The sharp crack of a snare drum, shuffling along at an insistent martial clip, is what first kicks “Civil War” into gear. The beat is joined by some menacing electronic bloops and nervous muttering, and while you may assume this is the work of some promising young bedroom producer, it’s actually a 1968 track, “Lovefingers,” by the radical duo Silver Apples. Somehow, the music matches the nervous, revolutionary energy on screen: the unlikely sight of an angry Brooklyn patrolled by troops, hundreds of people clashing in the streets, a suicide bomber putting an abrupt punctuation to it all. “Civil War” will remind you of the great combat films, the nauseating artillery ping of “Saving Private Ryan,” the surreal up-is-down journey of “Apocalypse Now.” It also bears a pronounced connection to the 2002 zombie road movie scripted by its writer-director Alex Garland, “28 Days Later,” a production that straddled the attacks of Sept. 11, 2001, and arrived in theaters scarred by timeliness. It’s the nowness of “Civil War” that will be much discussed. The movie takes place in an America that’s been amplified from its current