Just weeks ago, two decades of U.S. military engagement in Afghanistan came to a rather abrupt end.

Twenty years ago, at the start of the so-called war on terror, the strategic impact of the Sept. 11 attacks was immense both for U.S. national security policy and the Japan-U.S alliance.

At the time, the U.S. defined the threat of terrorism as a new type of war in the 21st century. As the nature of the threat was asymmetrical and unconventional, it required a major transformation in how the U.S. military operated. As a result, defense spending increased at a significant scale during the George W. Bush administration, both in terms of overseas contingency operations and expanded investments in areas such as counterinsurgency.