The official reason for the 2003 invasion was that Saddam Hussein was cooperating with al Qaida terrorists and had WMDs but would not cooperate with UN weapons inspectors. It's true Hussein did block UN inspectors, as he had done a few times before, but by late 2002 he agreed to let them in again, about a month after the US passed its Iraq war resolution that authorized the president to use military force to enforce the weapons inspections. In other words, Hussein backed down and complied with international law.

However, Secretary of Defense Rumsfeld wanted to demonstrate the strategy of "transformation", where a technologically advanced US military could project great power with far fewer GIs than normal. Vice-President Cheney had long wanted to oust Saudi oil power by installing a US-friendly regime in Shiite Iraq and eventually having it take over the oil fields in eastern Saudi Arabia, where the majority of population was also Shiite, unlike the Saudi royal family, which was sunni. The President relied heavily on them for foreign policy expertise because he had none himself. The latter made the president vulnerable to bad information, and Cheney just happened to control the flow of intelligence to the president and made sure it reflected Cheney's own biases. So the combination of 2 of Washington's most savvy operators wanting war in the Middle East working for an ignorant president who needed to prove his manhood made war highly likely.