According to military terminology, a two-front war occurs when opposing forces encounter on two geographically separate fronts. The forces of two or more allied parties usually simultaneously engage an opponent in order to increase their chances of success. The opponent consequently encounters severe logistic difficulties, as they are forced to divide and disperse their troops, defend an extended front line, and is at least partly cut off from their access to trade and exterior resources. However, by virtue of the central position, they might possess the advantages of the interior lines.

The term has widely been used in a metaphorical sense, for example to illustrate the dilemma of military commanders in the field, who struggle to carry out illusory strategic ideas of civilian bureaucrats, or when moderate legal motions or positions are concurrently opposed by the political Left and Right. Disapproval and opposition by the domestic anti-war movement and civil rights groups as opposed to the bloody military struggle of the late Vietnam War has also been described as a two-front war for the US troops, who fought in Vietnam.