What constitutes imperialism? According to Chalmers Johnson, the United States has an "imperial basing policy:"

As but one striking example of imperial basing policy: For the past sixty-one years, the U.S. military has garrisoned the small Japanese island of Okinawa with 37 bases. Smaller than Kauai in the Hawaiian Islands, Okinawa is home to 1.3 million people who live cheek-by-jowl with 17,000 Marines of the 3rd Marine Division and the largest U.S. installation in East Asia -- Kadena Air Force Base. There have been many Okinawan protests against the rapes, crimes, accidents, and pollution caused by this sort of concentration of American troops and weaponry, but so far the U. S. military -- in collusion with the Japanese government -- has ignored them. My research into our base world resulted in The Sorrows of Empire: Militarism, Secrecy, and the End of the Republic, written during the run-up to the Iraq invasion.

So: The United States makes agreements with the legitimately elected, democratic government of Japan, rather than with activist groups, and this constitutes an "imperial basing policty." By the way, I like the language of this paragraph - by making an agreement with the government of Japan, the US becomes in collusion with it.