American democracy seems to be teetering on the brink of collapse, and writer/director Alex Garland exploits that feeling for unbearably despairing suspense and censure with Civil War, a vision of a near-future in which the United States is no longer united. Less interested in devising one-to-one parallels with our present than with laying bare the catastrophic consequences of the division, hatred, and apathy that currently run amok from coast to coast, the Annihilation and Men auteur’s latest, in theaters April 12, is a portrait of individual and societal breakdown that plays like a companion piece to his 28 Days Later, except that this time around the vicious monsters are all of us. Spearheaded by a poignant Kirsten Dunst…