When US defence secretary James Mattis set out Washington's new national defence strategy earlier this month, he signalled the most radical shift in America's approach to global conflict since the September 11 attacks in 2001.
The worst-ever terrorist attack on the American mainland not surprisingly had a profound impact on US military policy. With the exception of the brief – and woefully under-resourced – military campaign to overthrow Iraqi dictator Saddam Hussein in 2003, the world’s most powerful military superpower found itself reduced to fighting an array of ragtag Islamist militant groups such as Al Qaeda, the Taliban and Islamic State of Iraq and the Levant (ISIL).
These low-intensity, counter-insurgency campaigns were primarily conducted on the basis of having lots of troops and special forces engaged in endless skirmishes with disparate groups of extremists, and rarely required the Pentagon to call on the services of the highly sophisticated weaponry at its disposal, such as its mighty aircraft carrier battle groups and fleet of heavy bombers.
But with the main military effort to defeat ISIL in Iraq and Syria now drawing to a close, Mr Mattis has signalled a dramatic shift in the Pentagon’s approach to future threats.
Instead of focusing its main effort on fighting global terrorism, the new strategy identifies China and Russia as presenting the “central challenge” facing the US military.
“Inter-state strategic competition, not terrorism, is now the primary concern in US national security,” the document states. The “long-term competitions with China and Russia” should now become the Pentagon’s principal priorities and meeting these challenges will “require both increased and sustained development”.
In short, rather than fighting small, messy wars against Islamist extremists and other terror groups, the US should prepare for the far more challenging state-on-state confrontations that might arise in the future, where the full spectrum of new defence capabilities, from cyber to a new generation of tactical nuclear weapons, will be needed to defend the interests of America and her allies around the globe.
The first challenge facing the new doctrine, which Mr Mattis discussed fully with US president Donald Trump and the rest of the national security team before making it public, will be to do battle with Congress to obtain sufficient funding to pay for this ambitious programme.
Firing a warning salvo to lawmakers, Mr Mattis criticised Congress for not providing the Pentagon with adequate funding and made the controversial claim that the constraints placed on defence spending by the Budget Control Act, as well as other resolutions passed by Congress in recent years, had inflicted more harm to the US military than any enemy in the field.
“We have no room for complacency,” Mr Mattis warned, “and history makes clear that America has no pre-ordained right to victory on the battlefield.”
The new American doctrine certainly reflects the rapidly changing global security environment which has been transformed by the emergence of Russia and China as major military threats, together with the long-standing challenges that continue to be posed by rogue states such as Iran and North Korea.
Under Russian president Vladimir Putin, Moscow has embarked on a massive programme to re-equip its military with the latest technology. Moreover its recent military adventures in Georgia, Crimea and Ukraine, not to mention its recent military intervention in Syria, have demonstrated that the Kremlin is not afraid to apply its newfound military might when circumstances permit.
China, too, is in the midst of a massive programme of investment in its military, building aircraft carriers and developing a new fleet of state-of-the-art jet fighters. China’s recent actions in the South China Sea, where it has transformed a network of reefs into operational military bases, have also demonstrated the new mood of military assertiveness that has taken root in Beijing.
The result is that, for the first time since the end of the Cold War in 1989, the US can no longer assume that its military will be able to undertake future combat operations in an uncontested military environment.
As Mr Mattis commented when setting out the new strategy: “Our military is strong, yet our competitive edge has eroded in every domain of warfare – air, land, sea, space and cyberspace – and it continues to erode.”
Then there is the threat posed by rogue states like Iran and North Korea, which are continuing to work on developing long-range ballistic missiles capable of carrying nuclear weapons.
But while Mr Mattis and the rest of the US national security establishment are right to focus on the new threats posed by the changing global geopolitical landscape, it is also important that they do not make the same mistake as the Obama administration and think that the war against Islamist extremists is at an end.
Barack Obama's decision to withdraw American forces from Iraq in 2011 was one of the main factors that resulted in ISIL seizing large tracts of the country in 2014 and establishing its so-called caliphate.
The success of the recent campaign to defeat ISIL might lead some western policymakers to believe the war against the terrorist group is over, which would be a grave miscalculation. ISIL might have suffered significant setbacks but its determination to attack the West and allies by any means possible remains as strong as ever.
So making sure that Islamist terror groups are denied the ability to launch large-scale attacks against western targets should remain as important a priority for the Pentagon as having the ability to wage state-on-state war.
Con Coughlin is the Daily Telegraph’s defence and foreign affairs editor