The United States has been Japan's most important ally since World War II and the U.S. nuclear umbrella of the Cold War came to define their security alliance.

Japan's role in the alliance has grown in prominence since the 1990s, both in terms of politics and security, especially after Prime Minister Junichiro Koizumi took office in April 2001.

Leaders of both nations now view the alliance in a more global context, and Koizumi and President George W. Bush are expected to reaffirm the strengthening of these ties during their summit Wednesday in Kyoto.