> He cannot just "decide to take Britain to Iraq", since the UK Government cannot start a war without Parliament's approval. Declaring war and deploying troops

> He cannot just "decide to take Britain to Iraq", since the UK Government cannot start a war without Parliament's approval. Declaring war and deploying troops is a power held by the Crown (which will do it on request of its government). Parliamentary approval is not required, parliament merely can hold a no-confidence vote and remove the government afterwards if it disagrees. The Afghanistan war just a bit earlier was debated in parliament, but not actually voted on by parliament. For Iraq, there was a formal vote for the first time, but again, not a formal authorization that would have had legal force - although loosing it realistically would have collapsed Blairs government once he committed to having the vote. (And also made any Labour MPs voting against it not just the MPs who were against the war, but also the MPs who forced their government to fail).