By far the most significant UK political event yesterday was the speech by outgoing US Defence Secretary Robert Gates.

His warning to European allies that they risked military irrelevance, and that the US could not perpetually be relied upon to fund European security was blunt and timely.

Of the many surprises about the way the media covers the coalition, one has been the relative ease with which the government has cut defence spending at a time of substantial military engagement. UK defence chiefs will be hoping David Cameron takes note of Mr Gates’ remarks. A lot of them saw the recent strategic defence review as being more about deficit reduction than defence strategy.

I am sure Britain will not have been anywhere near the top of Mr Gates’ list of irritating European nations. We remain one of the few major defence powers. But he will have had all countries in mind when he complained about the fall in spending, and the confused nature of European defence.

A few weeks ago, the Obama state visit suggested something of a continuing love in between our two countries. But Gates will have been speaking with President Obama’s approval. And he will have been speaking very much with the Obama generation in mind when he said that ‘if current trends in the decline of European defence 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.’

This is not a new threat or argument but Gates is right in saying that as the cold war recedes in the collective memory, the political circumstances for defence will change.

My next volume of diaries, out next month, starts in the midst of the Kosovo conflict and I remember from that period Bill Clinton and Tony Blair feeling many of the frustrations Gates expressed yesterday. Clinton was an internationalist, and a master politician, but even he found it hard to win support and understanding for a major US contribution to an effort on Europe’s doorstep. I remember too Clinton’s frustration that Europe looked so much to the US to do the heavy lifting.

That frustration is clearly there today with regard to Libya. ‘The mightiest military alliance in history is only 11 weeks into an operation against a poorly armed regime in a sparsely populated country,’ said Mr Gates. ‘Yet many allies are beginning to run short of munitions, requiring the US, once more, to make up the difference.’

America now contributes three-quarters of Nato military spending – much higher than at the height of the cold war, much higher than during Kosovo. It is hard to see how the post-Gates generation of political leaders will be allowed, able or willing to sustain that.

David Cameron is said to have taken a lead in pushing for action in Libya, action which would not have had any chance of success without the US. But though all Nato members voted for it, less than half have participated and less than a third have actually taken part in military strikes.  The fall in EU defence spending over the last two years is £45 billion . To give you a sense of the scale, that is the same figure as the entire defence budget for Europe’s biggest and richest country, Germany, which is sitting Libya out.

Mr Cameron is by nature a Eurosceptic. But he could do worse than trying to take on the challenge of reforming European defence. He has made a start with the French, who are never easy on the subject. But what the Gates speech showed up is the scale of a challenge still to be avowed in Europe, let alone met.