Writers and other members of the intelligentsia who advocate war or imperialism.
How the war fevers raged in those days after Sept. 11. The nation's syndicated belligerati were beside themselves. Columnist Michael Kelly flayed the unconscionable pacifists as pro-terrorist and evil. Charles Krauthammer argued for bombing an enemy city, anywhere.
Michael Powell, "An Eminence With No Shades of Gray," The Washington Post, May 5, 2002
Both men have retained their reputations as enfants terribles, probably perpetuated by their cruel-lipped, scowly poses in publicity photos.
Joy Press, "The Belligerati," The Village Voice, November 6, 2001
Over the past few months, however, the word's meaning shifted to the "warmongers/imperialists" sense thanks to a controversial article by the historian and novelist Tariq Ali that appeared in March:
[F]ormer critics of imperialism found themselves trapped by the debris of September 11. Many have now become its most vociferous loyalists. I am not, in this instance, referring to the belligerati Salman Rushdie, Martin Amis and friends ever-present in the liberal press on both sides of the Atlantic.
Tariq Ali, "The new empire loyalists: Former leftists turned US military cheerleaders are helping snuff out its traditions of dissent," The Guardian, March 16, 2002
This word began it linguistic life referring to writers who use an angry, confrontational style (belligerent + literati). Its first media appearance came in a review of the books The War Against Cliché, by Martin Amis, and Letters to a Young Contrarian, by Christopher Hitchens (see the review title).
Note that this sense of this term made an even earlier appearance on Usenet (April 10, 2001).