Few people hearing Tony Blair's words in recent days had any doubt that Britain was being prepared for war.

He has now confirmed that British forces have been involved in the US-led strikes against both Osama Bin Laden and Afghanistan's ruling Taleban regime.

His statement from Downing Street also made it crystal clear that this would be a long campaign in which Britain would play an increasingly important and hazardous role.

To use his own words, this is a moment of the utmost gravity for the world.

No one can predict how Bin Laden and his terrorist network will respond.

Alliance building

The only thing that seems certain is that, if they retain the ability to strike back, they will.

But - thanks in no small part to Tony Blair's diplomatic efforts - Bin Laden will be facing a world virtually united against him.

The prime minister has spent a large part of his time since the 11 September atrocities dashing from capital-to-capital in a concerted attempt to bolster the global coalition against Bin Laden and the Taleban.

He spoke then of the trap around Bin Laden being completed and most saw that as the clearest possible signal that action was imminent.

But the first signs of a change in gear came at last week's Labour party conference when Tony Blair's keynote speech was dominated by the international crisis.

War against terrorism

He left little room for manoeuvre in his threats to Bin Laden and the Taleban and all the signals from senior aides and ministers were that action was not only inevitable, but irrevocable.

The prime minister echoed many of his words from that speech when he addressed the nation from Downing Street as the attacks were underway on Sunday night.

He stressed this would be a long campaign fought on several fronts - military, diplomatic and financial.

He also repeated his statement that this was not a war against the Afghan people or Islam, but against terrorism and its supporters.

And he again carefully prepared people for the possibility of British military casualties.

Most agree that the prime minister has displayed genuine leadership and "grit" in this crisis.

The real test

He has stood shoulder-to-shoulder with President George Bush, as promised, and has taken Britain closer to the US than at any time since the Second World War.

There have been those criticising his apparent eagerness to provide America with its most uncritical ally.

But, at the moment, these voices are few and relatively muted.

He has stated that his military objectives are to "eradicate" Bin Laden and his terrorist network, and to "take action" against the Taleban.

That has inevitably raised questions over whether that means killing Bin Laden and bringing down the Taleban.

What is absolutely certain is that Tony Blair is facing the sort of terrible test that most prime ministers would hope never to face - but which has confronted all too many.

And, like virtually all wars, this one is unique and its outcome is uncertain.