The invasion and occupation of Iraq, which began in March 2003, was primarily led by the United States and included a coalition of several other countries. Here are the main participants:

1. United States: The leading force behind the invasion, citing the need to eliminate weapons of mass destruction (WMDs) and to combat terrorism.

2. United Kingdom: The second-largest contributor of troops, the UK played a significant role in the invasion and subsequent occupation.

3. Australia: Contributed troops and support to the coalition forces.

4. Poland: Provided troops and took command of a multinational division in Iraq.

5. Spain: Initially sent troops but withdrew them in 2004 after a change in government.

6. Italy: Contributed troops and was involved in the post-invasion stabilization efforts.

7. South Korea: Sent troops to support the coalition, primarily for reconstruction efforts.

8. Japan: Provided non-combat support, including humanitarian assistance and reconstruction efforts.

9. Denmark, Netherlands, and other countries: Several other nations contributed smaller numbers of troops or logistical support.

The coalition was often referred to as the "Coalition of the Willing," and while the U.S. and UK were the most prominent members, many other countries participated to varying degrees. The invasion was controversial and faced significant opposition both domestically and internationally.