Decision a political headache

Sending troops into combat is one of the heaviest responsibilities to be borne by a Prime Minister and one which Tony Blair has taken, as he and his predecessors have always done, in an appropriately sombre mood.

Mr Blair, it goes without saying, will have been kept in the closest possible touch with the build-up to today's action. But the tough political decision - to send the Marines to some of the most dangerous fighting terrain in the world - was taken weeks ago.

Unusually for New Labour, details of the mission were kept fully under wraps until the day last month when Defence Secretary Geoff Hoon informed MPs that 1,700 troops, led by 45 Commando, were on their way to fight alongside the Americans.

The timing of the operation was highly significant. It came after both Britain and the US had embarrassingly declared that the military operation against the al Qaeda and the Taliban was effectively over, following the Northern Alliance's sweeping successes in the autumn.

Mr Blair's official spokesman, in December, claimed the Taliban was a "spent force" - an assertion which has an increasingly hollow ring now. Hardline US defence secretary Donald Rumsfeld announced Operation Anaconda in which American troops would squeeze the last remaining life out of the enemy. But after weeks of fierce fighting, Anaconda abruptly stopped and, after a series of phone calls between President Bush and Mr Blair, the Marines were called in.

The Evening Standard was among the first to report, last Friday, that the Marines would be in action within days. Most Cabinet ministers then knew the barest minimum - Mr Hoon merely told them on Thursday that the "interim operational phase" was being entered.

No politician can do anything other than publicly express unqualified support when British troops go into combat. However, behind the scenes there are clear concerns at Westminster.

Tories share the worries of senior military officers that the armed forces are in danger of serious overstretch, with various operations being carried out in the Balkans, Sierra Leone and Northern Ireland, among others.

There are also fears that the work and safety of the separate British peacekeeping force, based in Kabul, will be compromised by the fierce fighting being undertaken by their comrades in the mountains.

Labour MPs, meanwhile, worry that the action can only worsen the situation in the powderkeg Middle East. In particular, they are massing behind serious opposition to US-British action against Iraq, openly clashing with Mr Blair's hardline stance.

The motivation of many of them is based on Old Labourstyle anti-Americanism, but among the 130 or so who have signed a key Commons motion rejecting calls for action against Saddam Hussein are several heavyweight figures, including former ministers. It is not just the awkward squad.

Mr Blair is fond of using colourful and aggressive rhetoric in the Commons when he stresses the need to take decisive action against terrorism and rogue regimes.

But behind him the silence and grim faces among his MPs tell a very different story - and one which has landed the Prime Minister with one of the biggest political headaches of his second term.