In the immediate aftermath of the Sept 11, 2001 attacks, the United States and its allies launched a "War on Terror" that began with the invasion of Afghanistan to oust the Taliban government and capture or kill Al-Qaeda leader Osama bin Laden.
This was followed by the invasion of Iraq in 2003 to depose dictator Saddam Hussein over his alleged support for terrorism.
The War on Terror has since been expanded to other parts of the world, most notably Africa, against Al-Qaeda and other terror groups such as Boko Haram and Al-Shabaab.
The campaign has yielded some notable successes, such as the killing of Osama and most of Al-Qaeda's top leadership. But the group continues to attract new followers around the world who have staged so-called "lone wolf" attacks that are difficult to prevent.
S-E Asia terror strikes
Terrorism has reared its ugly head in South-east Asia as well. Militant groups include Jemaah Islamiah, which wants to create an Islamic state in South-east Asia; the Moro Islamic Liberation Front, which seeks an autonomous region in the Philippines' Mindanao; and Abu Sayyaf, also in the Philippines.
They have staged notorious attacks like the Bali bombings in 2002 that killed 202, the siege of the Philippine city of Marawi in 2017, and a spate of suicide bombings in Indonesia in recent years.
Axis of evil
First used by then US President George W. Bush, the term points the finger at Iraq, Iran and North Korea as state sponsors of terrorism and countries pursuing weapons of mass destruction, leading to the imposition of crippling sanctions.
The definition of such an axis was in essence an attempt to rally the American populace in support of the war on terror, even though countries like Iran and Iraq were enemies at that point of time.
Notably, none of the terrorists involved in the 9/11 attacks were citizens of these states.
Rise in radicalisation
Not everyone is in favour of the war on terror. Critics note that military actions have destabilised the countries where they sought to eradicate terrorism, leading to the radicalisation of more people and the creation of newer groups.
The war on terror also legitimised the use of torture by the US at the Guantanamo Bay detention facility for terrorists and the Abu Ghraib prison in Iraq.
With the hardline Taliban once again in control of Afghanistan, there are fears that the country may once again be used as a launch pad for terror attacks.
Increased flight security
The 9/11 attacks transformed airport security around the world.
For example, people sending off travellers are now required to say goodbye long before security checkpoints. Passengers have to place their electronic devices in trays for scanning; coats, shoes and sometimes even belts are required to be removed for scanning.
Limits have also been imposed on liquids that can be taken on board, and carry-on luggage is scrutinised carefully.
There were changes on board planes as well. Metal cutlery has been replaced by plastic ones. Cockpit doors have been reinforced to prevent hijackings and are kept locked, and sky marshals in plainclothes are often on board flights to handle untoward situations.
Acts of violence against Muslims broke out following the 9/11 attacks and the bigotry continues to this day. There has been increased anti-Muslim sentiment across much of the world, with racial profiling occurring on a large scale.
The deeper anti-Muslim bias has seen the rise of right-wing movements in the US and Europe, as well as elsewhere, with politicians capitalising on the sentiment with anti-Muslim and anti-migrant policies.
Snooping on civilians
Less than two months after the 9/11 attacks, the US government passed the Patriot Act, permitting the use of government security apparatus to monitor civilians' phone and e-mail communications.
In 2013, American intelligence contractor Edward Snowden revealed the existence of wide-ranging information-gathering systems for spying on civilians in the US and foreign nationals, showing the full extent of the US snooping. This triggered an uproar at home and abroad.
More recently, reports that documented the use of sophisticated Israeli spyware Pegasus in multiple countries heightened concerns of government abuses in the name of surveillance.
Trust in media falls
In the wake of the 9/11 attacks, many US journalists put a patriotic spin on the news to pander to the public sentiment at the time.
For example, editorials were written championing war and citing anonymous sources to push the Bush administration's claim that Saddam had weapons of mass destruction. When none were found, public trust in the media took a massive hit.
In more recent times, the world has seen misinformation campaigns and fake news being peddled to a public that no longer trusts the mainstream media.
ISIS caliphate crushed
In 2014, the world's attention turned to the Islamic State in Iraq and Syria (ISIS), which conducted multiple attacks in the US, Europe and other regions.
Some of its biggest strikes include the 2015 Paris attacks that left 130 dead, and the bombing of a Metrojet flight from Egypt to Russia that killed all 224 on board. It has also conducted or inspired smaller attacks around the world.
It took five years before ISIS' caliphate was crushed by a coalition of 83 countries.
But while ISIS leaders like Abu Bakr al-Baghdadi have been killed, the group has since established a presence in other countries like the Philippines, Malaysia, Indonesia and India.