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.
Getting rid of the SCUDs was part of the disarming process. Obviously they did not get rid of them. Which leads to further proof that Iraq did not follow the UN resolutions. Therefore, the war is legal.
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.
Remember the Gulf War? Iraq agreed to disarm. They obviously did not. Hence 1441 reactivated 687, and we are back at war with Iraq again.
Does it matter if Iraq has WOMD or not? Not really, the fact is they didn't disarm and violated resolution 678, hence the legal right to attack and disarm them. Over 40 countries agree with this course of action.
Of course take a look at the weapons both sides have in the war:
[link|http://www.cnn.com/SPECIALS/2003/iraq/forces/weapons/index.html|http://www.cnn.com/S...eapons/index.html]
[link|http://news.bbc.co.uk/2/hi/in_depth/world/2003/military_fact_files/default.stm|http://news.bbc.co.u...files/default.stm]
[link|http://news.bbc.co.uk/2/hi/middle_east/2462583.stm|http://news.bbc.co.u..._east/2462583.stm]
But according to the UK prime minister's dossier on Iraq's weapons of mass destruction: "Iraq possesses extended-range versions of the Scud ballistic missile in breach of UN Security Council resolution 687 which are capable of reaching Cyprus, Eastern Turkey, Tehran and Israel. It is also developing longer-range ballistic missiles."
See the violation? See how clearly the fact got stated that these missiles violate resolution 678? What about that don't you understand?