A group of states that cooperate in an ad hoc or informal fashion, outside of more formal multilateral institutions and alliances. The term has been used recently to describe the group of countries supporting the 2003 U.S.-led invasion of Iraq, but its origins predate the George W. Bush administration. While the term usually refers to cooperation for military purposes, it has also been used in relation to other economic and human security issues in the Asia- Pacific region. A key component of any coalition of the willing seems to be its essentially pragmatic, ad hoc nature. The Oxford English Dictionary defines a “coalition” as “a union, combination or fusion”, noting that in politics the term often refers to “an alliance for combined action of distinct parties, persons, or states, without permanent incorporation into one body”. Webster's describes a “coalition” as a “combination, for temporary purposes, of persons, or parties, or states, having different interests”.

The precise origins of the term “coalition of the willing” are unclear, but a controversial 1992 Pentagon planning document captured the essence of the concept, arguing that coalitions “hold considerable promise for promoting collective action”. It said the United States “should expect future coalitions to be ad hoc assemblies, often not lasting beyond the crisis being confronted, and in many cases carrying only general agreement over the objectives to be accomplished”.

Recent usage continues to stress this flexible, impermanent character. Less than a month after the 9/11 attacks in 2001, U.S. Secretary of Defence Donald Rumsfeld said,