As the long-anticipated conflict with Iraq finally breaks out, BBC News Online world affairs correspondent Paul Reynolds answers key questions about the implications of war.

The simple answer is that the US and UK have decided that Iraq had not disarmed voluntarily as it was required to do by United Nations resolutions and that it is, therefore, going to be disarmed by force.

A more complex answer adds that the US wants to remove Saddam Hussein as a threat both to the region and potentially to itself and that it is asserting a right to "pre-emptive self defence".

Opponents of the war argue variously that the US has other motives as well - oil, domination of the Middle East or even finishing business left over from the Gulf War of 1991.

War will change the politics of the Middle East, though it should not change the map of Iraq. Iraq should survive as a unitary state.

The ambition of some in the Bush administration is that Iraq should be democratised, in part at least, and that this process should spread through the Arab and Islamic world.

Also at stake is the relationship between Islam and the West, for better or for worse. Some hope for a knock-on effect to improve the Israeli-Palestinian crisis. A long-delayed US-backed road map to Palestinian statehood is to be unveiled soon, but it will still involve long and difficult negotiations.

The future of the UN Security Council is also uncertain given the splits which have been exposed.

The removal of Saddam Hussein and his associates for a start. In due course it would like to see an Iraqi Government which might not be wholly democratic but in which the people would have a say, the press would be free and the army would be in barracks.

Cynics say that the US wants Iraqi oil as well, which is why Washington is taking steps to say that the oil will be used for the benefit of the Iraqi people. Delivering on that pledge will be a test of US actions.

Potentially it means a more democratic way of life, the lifting of all sanctions and the rebuilding of their country. For some it will mean foreign domination.

But, of course, all are having to go through a war to get there and some, therefore, won't even survive.

The British Attorney-General Lord Goldsmith has published his opinion that war is justified under Security Council resolutions. He says that the authority goes back to resolutions 678, which permitted war against Iraq over Kuwait and 687, which laid down terms for a ceasefire. These were reactivated by 1441 which warned of "serious consequences" if Iraq did not comply with instructions to disarm.

Other lawyers, however, say that the resolutions are too vague and ambiguous to permit military action and that a clear-cut resolution authorising this particular military action is needed from the Security Council.

Yes, given the level of firepower and technology available to the US and its allies.

The question is also whether victory can be achieved with minimum loss of life - civilians and soldiers.

A victory with heavy losses would be very different.

There will be two priorities. First, US and UK forces will need to get immediate aid to those who need it. Second, a UN mandated framework will need to be established so that aid agencies and outside bodies - such as the UN, the EU and the international financial institutions - can go in knowing they will be working within a legal framework and not for what may be seen as an occupation force.

An interim civilian administration will then be set up and operate until a representative administration takes over - at a time to be determined.

The Kurds in the North will want as much autonomy as possible. The Shias in the south rose up against Saddam Hussein in 1991 so they too will have their demands. The Sunnis in the middle are afraid that they will be squeezed, though Iraqi exiles sometimes say that the ethnic and religious differences between Iraqis can be exaggerated.

There has been talk among exiles and the US Government about putting him on trial in Iraq for war crimes. He might want to go down fighting.