WASHINGTON — Memorials to the war dead of the 20th century are among the central attractions in the nation’s capital. So it has always been notable that one of the most consequential American conflicts, World War I, lacked national recognition.

Now, as the United States withdraws from its longest war, a memorial that recognizes one of its most complicated ones officially opens in Washington on Friday, after years of tangling among preservationists, urban planners, federal officials and the commission that realized its creation.