To look at the front page of the New York Times of Tuesday, September 11th, 2001 is to reach back into another era – an era, in many ways, which doesn’t look very different to that of today.

The president was under pressure over the economy, there was violence in the Middle East and the New York Giants had lost to the Denver Broncos.

But even before many New Yorkers would have opened their newspaper on that clear, sunny September morning, America and the world had been changed forever.

At 8.46am and 9.03am, two hijacked airliners were flown into the twin towers of the World Trade Center. Another jet was crashed into the Pentagon near Washington DC and a fourth was brought down in a field in Pennsylvania.

What had seemed unimaginable – an unprecedented terrorist attack on American soil, striking at the very heart of society and witnessed on television screens around the world – was a shock the country has still not absorbed.

The results of a recent USA Today/Gallup poll are astonishing. Some 60% of Americans say the attacks permanently changed the way the country lives, more than the number who felt that way on the tenth anniversary.

The youngest, and those who weren’t even born on September 11th, felt that impact the strongest of all.

Twenty years on, the debate continues about how much America and the world was altered by the events of September 11th but in myriad ways, big and small, the scars of that day are still evident.

Endless War

Who would have thought that when the US launched airstrikes on Afghanistan within a month of September 11th that it would be almost 20 years before the last American troops would finally leave the country?

The initial aim of the invasion, ordered by US president George W Bush, was to crush al Qaeda and Osama bin Laden, the terrorist group and its leader blamed for planning and carrying out the attacks, and deny them the base from which they had operated. Prime Minister Tony Blair was a key ally of the US in offering military support.

But Mr Bush had already told the US Congress and the American people that the country was engaged in a new type of military action that went far beyond a few targeted strikes against a single enemy.

The “war on terror” was born and it would not end, he said, “until every terrorist group of global reach has been found, stopped and defeated”.

The war in Afghanistan became the longest in US history. Some 800,000 served there and nearly 2,500 died. More than 20,000 are listed as wounded – the true cost of psychological wounds is far higher.

The more than 400 UK service personnel who died in Afghanistan add to thousands of Afghan civilians, police and military personnel, aid workers and contractors over the 20 years.

While the war in Afghanistan enjoyed public support initially, that waned over the years, especially after the killing of bin Laden in 2011.

That the “war on terror” encompassed the far more controversial invasion of Iraq – in the supposed hunt for stocks of weapons of mass destruction that were never found – would cost a further 4,500 American military lives, some 179 British and 100,000 Iraqi. A million Americans served in Iraq.

Everywhere you look are remnants of the war. The prison camp at Guantanamo Bay just one we almost forget these days.

The pursuit of the “war on terror” would define American foreign policy and arguments rage about whether it was won or lost.

It is undeniable that the spectre of a repeat of September 11th, the fear of an attack on the homeland, has driven American actions abroad for far longer than anyone expected.

Air travel

Anyone who has flown into, out of or around the US in recent years will be familiar with those blue-uniformed custodians of the body scanner, the TSA.

Before September 11th, not only did the Transportation Security Administration not exist but airport security was a pale shadow of the operation we see today. Fewer than 10% of checked bags were screened back then.

The TSA was built from scratch within months and in direct response to the September 11th attacks. It is now a behemoth with a budget of $8bn and has undoubtedly made air travel safer.

The law that created it also mandated that all bags be screened, cockpit doors be reinforced and air marshals be put on planes.

If you can remember flying pre-2001, or if you watch an old film with an airport scene, it was a time of no lines at security, no need for a boarding pass to get to the departure gate and far less stress.

But as previously-unseen threats manifest, so too have security measures. Things that could be used as a weapon, like blades, were banned. Shoes had to be removed, a move that followed the failed shoe bomb attack in 2001, and electronics received extra screening.

The limit on liquids which could be used to make a bomb have been accepted, sometimes grudgingly, by passengers along with those growing queues and the need to arrive earlier at the airport.

While some success is obvious – 3,200 guns seized at airports last year, almost all loaded – much of the security infrastructure is hidden from view with vetting and background checks and behavioural analysis part of the system. This has also led to suspicions and complaints of racial profiling.

And like the booming business in trusted-traveller programmes – where passengers pay fees and disclose background information to bypass the checks – it has come at the cost of another big aspect of change in our post-September 11th world: privacy.

Surveillance and privacy

Just 45 days after the September 11th attacks, the Patriot Act was signed into law with the stated aim of tightening US national security.

It expanded the surveillance reach of law enforcement including permitting the tapping of international and domestic telephone lines. In essence, it made it easier for the US government to monitor US citizens.

Opponents say it was the birth of a “mass surveillance regime”, expanding powers to carry out electronic searches without court orders and property searches without someone’s consent or even knowledge.

In the years that followed, those programmes were expanded and supported by the Bush and Obama administrations and Congress.

The revelations of whistle-blower Edward Snowden in 2013 reverberated around the world, his allegations of the broad extent of the US National Security Agency’s efforts to gather data on a massive scale revealed the expansion of the power granted to the intelligence services.

Civil liberties groups began a fight against the scope of the laws arguing they undermined privacy rights and are, in some cases, unconstitutional.

But, as Congress quietly renewed many of the powers, public opinion remained broadly supportive of the intelligence services right to snoop in the name of national security.

A quarter of Americans, though, did say they had changed the way they used technology in the wake of the Snowden revelations.

Congress has now acted to rein in some of those powers and the more controversial data collection techniques have been abandoned.

But in an era when data is exploding, and with a greater awareness of transparency and privacy, the tension between civil liberties and national security is alive and well.

Anti-Muslim sentiment

In 2000, 12 anti-Muslim assaults were reported to the FBI in the US. In 2001, the number had leapt to 93. It has never returned to pre-2001 levels.

A decade and a half after September 11th, half of Muslims in the US said they found it more difficult to live in the country as a result of the attacks.

But it initially appeared the backlash against the Muslim community that everyone had feared could be averted.

Six days after the attack, President Bush visited a mosque in Washington and condemned harassment of the Muslim community. “The face of terror is not the truth faith of Islam,” he said. “That’s not what Islam is all about. Islam is peace. These terrorists don’t represent peace. They represent evil and war.”

Polls taken two months after September 11th showed 59% of Americans had a favourable view of Muslim Americans, up from the number before September 11th.

But in the years that followed, polls showed a growing suspicion of people of Middle Eastern descent and a growing number of Americans who associated Islam with violence.

Even though the Muslim population has grown in the years since September 11th, researchers say many Americans know little about Islam and that views about the Muslim community have divided along political lines.

A survey by Pew Research in 2007 found that half of Americans believe that Islam is “not part of mainstream American society”, but that view was held by 68% of Republicans and just 37% of Democrats.

American psyche and patriotism

It is one very visible testament to the impact of September 11th on every street in America

The flags that fly on porches and front lawns, the protocol of never leaving them there unlit after dark, gained an added meaning for many. There is also a greater suspicion of those who don’t fly the flag, who don’t wear their patriotism proudly in post September 11th America.

Millions of words have been written about the surge in patriotism after September 11th. President Bush harnessed the spirit, with a bullhorn in one hand and his arm around a firefighter at Ground Zero, to rally Americans around the flag.

It has often been said that the US military saw a surge in enlistment after September 11th. In fact, despite a surge in calls to recruiting centres, the increase in the number who actually signed up was negligible. In 2005, the US fell short of its annual recruitment goal.

But there is no doubt many of those who did enlist in 2001 and 2002 were motivated by a desire to seek revenge. And, after all, the US had not been actively engaged in an official war until the invasion of Afghanistan.

Bush’s exhortation that “you are either with us or against us” struck a chord.

In the immediate aftermath of September 11th, there was a surge in the number of people looking to volunteer for charities and donate blood. A similar rise in attendance was seen at churches.

When researchers looked at all of those numbers again nine months after September 11th, only the levels of patriotism remained as high.

This took root in American culture as even Hollywood focused on patriotism rather than violence.

And the overt reverence for the military and first responders and their service is an undoubted legacy of what Americans witnessed on September 11th.

Changed the world

While the ways in which September 11th changed America are unmistakable, the impact of those attacks around the globe is a varied picture of the subtle and brutal.

For the people of Afghanistan and Iraq, years of war and its terrible costs are a living embodiment of America’s reaction to the attack on home soil. The repercussions have been felt throughout their neighbours and beyond.

The loss of life of British military personnel, and those of other allied nations in those wars, are scars with which hundreds of families still live.

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If few people in the broader population paid attention to the names of al Qaeda and Osama bin Laden before September 11th, many countries have seen first-hand in the years since the devastation of the sort of attacks they inspire.

The world has also drawn lessons from the withdrawal from Afghanistan and whether the “war on terror” succeeded. Wherever we are in the world, even if it is something as minor as taking our belt off at airport security, the impacts of September 11th are with us.