Mr Donald Trump, the Republican Party's presumptive presidential nominee, has expressed deep scepticism about the value of America's alliances. His is a very 19th-century view of the world.
Back then, the US followed George Washington's advice to avoid "entangling alliances" and pursued the Monroe Doctrine, which focused on US interests in the Western Hemisphere. Lacking a large standing army, the US played a minor role in the 19th-century global balance of power.
That changed decisively with its entry into World War I, when then President Woodrow Wilson broke with tradition and sent US troops to fight in Europe. He also proposed a League of Nations to organise collective security on a global basis.
But, after the Senate rejected US membership in the League in 1919, the troops stayed home and America "returned to normal". Though it was now a major global actor, the US became virulently isolationist. Its absence of alliances in the 1930s set the stage for a disastrous decade marked by economic depression, genocide and another world war.
Ominously, Mr Trump's most detailed speech on foreign policy suggests that he takes his inspiration from precisely this period of isolation and "America First" sentiment. Such sentiment has always been a current in US politics, but it has stayed out of the mainstream since the end of World War II for good reason: It hinders, rather than advances, peace and prosperity at home and abroad.
The turn away from isolation and the start of the "American century" in world politics was marked by then President Harry Truman's decisions after World War II, which led to permanent alliances and a military presence abroad. The US invested heavily in the Marshall Plan in 1948, created Nato in 1949, and led a United Nations coalition that fought in Korea in 1950. In 1960, then President Dwight Eisenhower signed a security treaty with Japan. US troops remain in Europe, Japan and South Korea to this day.
While the US has had bitter partisan differences over disastrous interventions in countries such as Vietnam and Iraq, there is a bedrock of consensus on its alliance system. Opinion polls show popular majorities in support of Nato and the US-Japan alliance. Nonetheless, for the first time in 70 years, a major US presidential candidate is calling this consensus into question.
Alliances not only reinforce US power, but they also maintain geopolitical stability - for example, by slowing the proliferation of nuclear weapons. While US presidents and defence secretaries have sometimes complained about allies' low levels of defence spending, they understand that alliances are best seen as stabilising commitments - like friendships, not real estate transactions.
Unlike the constantly shifting alliances of convenience that characterised the 19th century, modern American alliances have sustained a relatively predictable international order. And yet Mr Trump extols the virtues of unpredictability - a potentially useful tactic when bargaining with enemies, but a disastrous approach to reassuring friends. Americans often complain about free riders, without recognising that the US has been the one steering the bus.
It is not impossible that a new challenger surpasses the US in the coming decades and takes the wheel. But it is not likely either. Among the features that distinguish the US from "the dominant great powers of the past", according to British strategist Lawrence Freedman, is that "American power is based on alliances rather than colonies". Alliances are assets; colonies are liabilities.
A narrative of American decline is likely to be inaccurate and misleading. America has many problems, but it is not in absolute decline and is likely to remain more powerful than any single state for the foreseeable future.
The real problem for the US is not that it will be overtaken by China or another contender, but that a rise in the power resources of many others will pose new obstacles to global governance. The real challenge will be entropy - the inability to get work done.
Weakening US alliances, the likely result of Mr Trump's policies, is hardly the way to "make America great again". The US will face an increasing number of new issues that require it to exercise power with others as much as over others. And, in a world of growing complexity, the most connected states are the most powerful.
Contrary to claims that the "Chinese century" is at hand, we have not entered a post-American world. The US remains central to the workings of the global balance of power and to the provision of global public goods. But its pre- eminence in military, economic and soft-power terms will not look like it once did. The US share of the world economy will fall, and its ability to wield influence and organise action will become increasingly constrained. More than ever, its ability to sustain the credibility of its alliances as well as establish new networks will be central to its global success.
• Joseph S. Nye Jr, a former US assistant secretary of defence and chairman of the US National Intelligence Council, is University Professor at Harvard University. He is the author of Is The American Century Over?
A version of this article appeared in the print edition of The Straits Times on May 17, 2016, with the headline 'Trump would weaken the US'. Print Edition | Subscribe
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