Last month, he visited China for the Group of 20 leaders' summit at the invitation of President Xi Jinping, and both leaders took stock of the broad and growing friendship, in particular the third government-to-government project in Chongqing.

PM Lee then went on to Laos for the annual Asean and East Asia summits involving leaders of all 10 South-east Asian countries and their key partners to take regional partnerships forward.

Later that month, he also visited Japan, met Prime Minister Shinzo Abe and witnessed the signing of several pacts to strengthen ties in trade, infrastructure and technology.

In the first week of this month, he made a trip to India where he met Prime Minister Narendra Modi, and witnessed the signing of several pacts to strengthen cooperation in intellectual property and skills training.

Last week, PM Lee was in Australia where he met Prime Minister Malcolm Turnbull, witnessed the inking of pacts to step up defence, trade, innovation and security ties, and made a historic speech to its Parliament, the first Singapore prime minister to do so.

As he said in a Twitter post yesterday on his visit: "We've signed several agreements with Australia to take our relationship further. Happy to call them 'mates'."

In Canberra, he told Singapore reporters there are still a few more trips to come. This year saw more trips than the year before, when the calendar included SG50 celebrations and the general election.

But opportunities also presented themselves, such as the one-year anniversary of the strategic partnership with India and the conclusion of a package of initiatives under the Comprehensive Strategic Partnership with Australia, he said.

"While we must not lose sight of our own domestic problems, neither must we forget that there's an outside world. There are things happening that affect us, there are opportunities and friends we have to pay attention to," PM Lee added.

The need to build upon these friendships was a key theme in his National Day Rally speech in August, where he said it was important for Singapore to have a network of friends, in its neighbourhood as well as among big powers.

The US presence in this region has fostered peace and stability and enabled countries to grow, and China's stability and prosperity have greatly benefited Asia and the world, he said then.

"We are friends with both America and China. It is easiest to do this if the two countries are on good terms with each other. In fact, both countries do aim to be on good terms with each other," he added.

"Both believe the Pacific is vast enough to accommodate both powers and President Xi Jinping said recently that America and China should 'cultivate common circles of friends'. That is precisely what Singapore is trying to do - to be among America's circle of friends, and also among China's circle of friends."

These friendships are not just about geopolitics, but economics too. Singapore has significant trade accounts with the US, China, Japan, India and Australia, and in its key partnerships, both sides have sought to advance common interests and mutual benefits.

KEY MESSAGES

On his visit to Washington, PM Lee made the point that America's staying open to trade with the region and its presence in the Asia-Pacific have helped create the basis for a peaceful, rules-based order, and he expressed the hope the US would stay engaged in this region.

In an interview with Chinese magazine Caijing in Hangzhou, he said that countries in the region should also determine their own path and not be divided by big powers.

"Countries in Asia, Singapore, certainly, but many other countries too, are good friends to both China and America and we would like to be good friends with both.

"And this is easiest if both China and America are working well with each other," he said.

He also said: "If Asean is split and South-east Asia becomes a region where different powers contend with each other and try to jockey and gain advantage and play one country against another, it will raise tensions in the region...

"It will not be to the advantage of the powers either, because it would mean a less stable Asia."

These remarks echo positions that have been consistently held by Singapore's top diplomats, who say they remain ever more crucial as the global balance of power shifts and relations between the US and China become more interlinked, but also more competitive.

Singapore's interests are best served by all players taking an active role in upholding peace and stability in the Asia-Pacific.

A key message PM Lee had for his friends in Tokyo, New Delhi and Canberra respectively was that a Japan, India and Australia that remain active and engaged in the region - trading more with it and taking part in maintaining its security - was good for the region overall.

As he told a special session of the Nikkei International Conference on the future of Asia in Tokyo: "All sides have a vested interest in reaching a new and workable balance, and in minimising conflict.

"For if countries fail to work together, we are not just losing opportunities to prosper together, but are also putting at serious risk all that we have achieved so far."

But a more challenging environment ahead, observers say, also makes it important for Singaporeans to be aware of the key elements of their country's foreign policy.

SINGAPORE'S FOREIGN POLICY AIM

The overriding objective of Singapore's foreign policy is similar to why the country sets great store by defence and a strong, credible Singapore Armed Forces: to ensure Singapore's survival and sovereignty as an independent nation.

The country often describes its own foreign policy as principled.

Beyond building ties with partners, it seeks to foster common interests among friendly nations so as to uphold a stable, secure region and a global order that abides by the rule of law.

Singapore diplomats and observers say these principles are few but fundamental. They include:

•Upholding international law and supporting a rules-based international order

•Staying committed to an open economy that depends on freedom of navigation in international maritime and air space

•Seeking peaceful resolution to disputes

•Being a reliable partner who respects treaties and contracts

•Adopting a realistic worldview while remembering that Singapore is a price-taker and not a price-setter in international affairs

Adherence to these principles is not just about doing the right thing. It is a matter of survival, they add.

As Professor Chan Heng Chee, a veteran diplomat, says: "As a small country, these principles are our best defence and win us respect.

"It is important to retain them because our actions are then consistent and based on reason. They are important for our self-preservation. Other countries see us as reliable and constant."

These guidelines have served Singapore well over the last 50 years.

Singapore was one of the five founding members of Asean in 1967, set up at the height of the Cold War to prevent larger outside powers from dominating the region.

Asean has fostered peace and stability in the region and created a climate where the ties that bind its various members individually and as a group are stronger than the disagreements they may have from time to time. And Singapore has reiterated the need for the grouping to be united and in control of events in its region, even as its unity has been tested on occasion.

Today, one foreign policy challenge that makes the headlines is territorial disputes in the South China Sea involving four Asean members. Singapore is not a claimant state, but the significance of the issue and Singapore's position on it prompted PM Lee to address it in his National Day Rally speech.

He reiterated Singapore's stance on respect for international law, freedom of navigation and the need for a united Asean. He also said Singapore took no sides on the overlapping territorial claims and was simply sticking to its own principled, consistent stance.

CHALLENGES AHEAD

In his rally speech, PM Lee noted that "life is never so straightforward" as Singapore has had issues with its friends from time to time. Also, the interests of its friends will sometimes conflict, and Singapore will be pressured to choose sides.

Singapore has all along maintained its right to determine its own course of action and advance its people's interests, standing its ground in the face of pressure from friends. In 1994, American teenager Michael Fay was caned for vandalism, despite pressure from US officials to drop the punishment.

Singapore has also had periodic run-ins with Jakarta officials over the issue of transboundary haze emanating from fires in Indonesia.

"It's not because we love confrontation. We just stand our ground because we are a small country and need to hold on to certain principles," says senior fellow William Choong of the International Institute for Strategic Studies-Asia.

Last month, Chinese state-linked newspaper Global Times accused Singapore of pushing to include an international tribunal's ruling on the South China Sea - that dismissed China's territorial claims there - in the final document of the Non-Aligned Movement summit.

The article sparked a war of words with Singapore's Ambassador to China Stanley Loh, who disputed the paper's account and said there was a common, united Asean position to update the document.

Despite the clarification, there was some public backlash against Singapore in China.

The incident was seen as a move by some to sway Singapore's position on the subject. It was also seen as an example of increasingly complex challenges Singapore faces as the global balance of power shifts.

In such a new and evolving global order, can Singapore's longstanding foreign policy - to be friends with all sides and take no sides - continue to be effective?

Shortly after the Global Times saga began, some commentators were quick to declare that China had embarked on a more coercive approach towards Singapore.

But outspoken career diplomat Bilahari Kausikan thinks Beijing had begun being more assertive earlier. He sees the Global Times episode as "being played out loudly in public as part of an ongoing attempt to bypass the Government and influence policy by making Singaporeans jittery".

"To understand the game is the first step in not being forced to play it," Mr Kausikan added.

Indeed, Mr Lee acknowledged in his National Day Rally speech that some Singaporeans with business and other professional partners in China are worried that any tension will hurt their dealings there.

"We want good relations with other countries if it is at all possible, but we must also be prepared for ups and downs from time to time," he said, adding that the Government had to take a national point of view and defend Singapore's overall interests.

Besides, the country's ties with China go beyond the South China Sea issue and are underpinned by decades of relations and collaboration on major projects, he added.

MP Vikram Nair, who chairs the Government Parliamentary Committee for defence and foreign affairs, says some businessmen have expressed concern about the situation. But they understand why the Government was adopting such a stance on the issue, he says.

"Both sides are practical and there are many things we are working on together," Mr Nair adds.

REPUTATION TO PROTECT

Observers say Singapore is respected as a voice of reason abroad, thanks to its reputation of consistency and trustworthiness.

Its stature was built by decades of active participation in international bodies such as the United Nations, and by leading efforts such as starting the Forum of Small States, for nations with populations below 10 million, in 1992.

Because Singapore is not beholden to any country for aid, it is able to speak its mind and take positions of principle. As Dr Choong says: "Singapore punches above its weight in the diplomatic space."

Singapore's diplomacy has also come under the spotlight as it is country coordinator for Asean-China Dialogue relations till mid-2018.

Many in China believe that Singapore, with its large ethnic Chinese community, is a Chinese society that should understand and support Chinese interests.

"China expects special treatment of Chinese sensitivities and interests. There are no two ways about this," says S. Rajaratnam School of International Studies executive deputy chairman Ong Keng Yong, a veteran diplomat.

Such fundamental mismatches in the characterisation of bilateral relations are not new to Singapore.

For instance, some in Indonesia and Malaysia hold an abang-adik attitude towards the island state, treating ties as that between an older and younger brother.

But it does mean Singapore diplomats are confronted from time to time by casual remarks from Chinese counterparts on Singapore's Chinese-ness. The Singaporeans then politely but firmly remind them that Singapore is a sovereign, multiracial country, with English as its working language.

Likewise, Singapore's actions or positions in strengthening ties with other partners are not directed at any country. As PM Lee said in Canberra, the Singapore-Australia partnership is part of a network of relationships that is about strengthening regional stability, by building "an open and inclusive regional security architecture".

Key to such an architecture are basics Singapore has called for - a commitment to international law and a rules-based global order.

It also entails seeking new friendships and reinforcing old ones.

The travel PM Lee has chalked up on behalf of the country goes a long way in maintaining Singapore's international standing and relevance to its partners, says Mr Ong.

"Some of my friends living and working overseas stress to me the significance of PM Lee visiting the countries where they are in," he says. "They reminded me Singapore faces stiff competition from other countries with cheaper things to sell and more talented expertise to share. Without PM Lee's high-profile foreign visits, the goodwill and opportunities for Singapore cannot be fully tapped."

PM LEE'S TRIPS IN PAST THREE MONTHS

JULY 31-AUG 5: UNITED STATES

PM met President Barack Obama and was hosted to a state dinner at the White House. Mr Lee also spoke of the importance of the Trans-Pacific Partnership trade pact to America and the region.

AUG 25: INDONESIA (POSTPONED)

PM was to attend the Singapore-Indonesia Leaders' Retreat in Semarang and open the Kendal industrial park with President Joko Widodo. But he took ill at the National Day Rally and was given a week's medical leave. The retreat is scheduled to take place next month.

SEPT 2-5: CHINA

PM met President Xi Jinping and attended the G-20 summit in Hangzhou at Mr Xi's invitation. He also visited Chongqing for an update on the third Sino-Singapore joint project.

SEPT 6-8: LAOS

PM attended the Asean and East Asia summits. He also had introductory meetings with several regional leaders.

SEPT 26-29: JAPAN

PM was hosted to lunch by Emperor Akihito and met Prime Minister Shinzo Abe. He also delivered a speech at a special session of the Nikkei International Conference on the Future of Asia, where he spoke about China and Japan and the need for stability in the Asia-Pacific.

OCT 3-7: INDIA

PM met Prime Minister Narendra Modi and both leaders agreed to set up a committee to facilitate investments between both countries. They also agreed to expedite a review of an existing trade pact and expand cooperation in several areas. PM also visited Udaipur for the launch of a skills training institute.

OCT 11-13: AUSTRALIA

PM addressed the Australian Parliament in a speech that paid tribute to the deep, lasting ties and the shared strategic outlook and social ethos of both countries. He also witnessed the signing of agreements to deepen cooperation in defence, trade, innovation, law enforcement and the arts.