US defence chief: Europe may no longer be worth defending

Looking first at Afghanistan, he noted that the operation got bogged down in national caveats which hampered the range of actions European soldiers were allowed to do - a hint at Germany's restrictions on its troops to use lethal force, which meant that they could not be deployed in combat against the Taliban.

As for troop withdrawal, with the Netherlands suddenly pulling out after a government was forced to resign last year over the Afghan war, Gates said: "We cannot afford to have some troop-contributing nations to pull out their forces on their own timeline in a way that undermines the mission and increases risks to other allies. The way ahead in Afghanistan is 'in together, out together'."

Nato's flagship mission in Afghanistan has also "exposed significant shortcomings" in the military alliance - both concerning capabilities and "political will."

"Despite more than 2 million troops in uniform – not counting the US military – Nato has struggled, at times desperately, to sustain a deployment of 25,000 to 40,000 troops, not just in boots on the ground, but in crucial support assets such as helicopters, transport aircraft, maintenance, intelligence, surveillance and reconnaissance, and much more," Gates said.

When it comes to the more recent mission in Libya, similar shortcomings made it "painfully clear" that they can undermine the success of a campaign that has broad political support, no troops on the ground and is "a mission in Europe's neighbourhood deemed to be in Europe's vital interest."

"While every alliance member voted for the Libya mission, less than half have participated at all, and fewer than a third have been willing to participate in the strike mission. Frankly, many of those allies sitting on the sidelines do so not because they do not want to participate, but simply because they can't. The military capabilities simply aren't there."

He cited lack of intelligence, surveillance, and reconnaissance assets, which make even the most advanced European fighter jets useless. Italy's ambition to run the air campaign from a Naples-based headquarters in fact meant that "mainly US targeting specialists" had to be deployed there "to do the job."

"The mightiest military alliance in history is only 11 weeks into an operation against a poorly armed regime in a sparsely populated country – yet many allies are beginning to run short of munitions, requiring the US, once more, to make up the difference."

With more security-consuming European allies than partners able to shoulder the burden in Nato, Gates questioned the very rationale of this alliance, born out of America's willingness to protect Western Europe against a potential Soviet invasion during the Cold War.

"Some two decades after the collapse of the Berlin Wall, the US share of Nato defence spending has now risen to more than 75 percent – at a time when politically painful budget and benefit cuts are being considered at home," Gates said.

Frustrated by years of trying to convince European allies to step up their defence spending and upgrade their military capabilities, Gates projected a "dim, if not dismal future for the transatlantic alliance": that the US simply stops footing the bill.

"The blunt reality is that there will be dwindling appetite and patience in the US Congress – and in the American body politic writ large – to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defence."

"Indeed, if current trends in the decline of European defense capabilities are not halted and reversed, future US political leaders – those for whom the Cold War was not the formative experience that it was for me – may not consider the return on America's investment in Nato worth the cost," he warned.

Responding to this doomsday scenario, Nato spokeswoman Oana Lungescu said during a press briefing on Friday that this was in line with repeated calls from Nato chief Anders Fogh Rasmussen to improve military spending and make "smart" defence cuts where necessary.

"There is clearly a long-standing concern about the transatlantic gap in defence spending, there is a risk that European allies may even fall behind in terms of technological development. We all know there is an economic and financial crisis going on and when all parts of budgets are being cut, defence budgets cannot be exempt," she said.