By Daniel Dombey, US Diplomatic Correspondent

The man responsible for America’s military has come to the country at the heart of the US fight against militant Islamists – and yet he can’t talk about a key part of that struggle.

Robert Gates, US defence secretary, has followed up on his trip to India by flying to Pakistan – his first visit to the country for almost three years. The last time he came calling, in February 2007, was only a couple of months after he had been installed as Secretary of Defence by George W Bush.

The Obama administration has been more forthright than Bush’s team was about how Pakistan – a nuclear armed state of some 166m – is more important for US national security than is Afghanistan, the country in which the US will soon have 100,000 troops.

But what is lost in the talk of grand strategy is the fact that the US’s experience in the Af-Pak region consists of two wars. One, in Afghanistan, revolves around improvised explosive devices, the crude bombs that account for most of the Nato casualties at the hands of insurgents. The other, in Pakistan, is largely about the drone strikes that are the US’s weapon of choice against al-Qaeda and Taliban leaders.

How long those two conflicts will last may depend on large part on how long the US is willing to accept significant numbers of IED casualties, and how long Pakistan will tolerate the drone strikes on its terrain.

The problem is that as a CIA operation, using bases on Pakistani territory, with the reluctant and off-the-record approval of Pakistani authorities, the drone strikes are not anything that US officials like Gates can talk about in public.

That’s all the more important, since the drone strikes are deeply unpopular in Pakistan, and are blamed for many civilan casualties (the US often disputes just how many). The issue has grown in intensity after a Jordanian double agent recently killed seven CIA operatives in  Afghanistan, since when the tempo of the drone strikes has been upped still further.

So when asked about the drone strikes on the flight over Gates declined to talk about such operations. Instead he highlighted the US desire to avoid civilian casualties and Washington’s readiness to provide Pakistan with surveillance vehicles, adding a bland and not hugely meaningful: “We are very mindful of Pakistan’s sovereignty.”

It’s far from clear how effective such bromides will be in diluting opposition to the strikes. After all, all such attacks depend on a green light from Islamabad. Almost any country would be wary of allowing another power to carry out systematic, repeated attacks on its own territory – and that’s even more the case in Pakistan, where anti-American sentiment runs strong and the government is weak.

One gets the sense that the status quo can’t last for all that long.

But  if you are looking for a public defence of Washington’s most controversial actions in its war against al-Qaeda – its most direct strikes against  Islamist groups – sorry, the US can’t help.