War is not without economic benefits. These are not limited to having misfortune strike trade rivals. At certain historical times and places, war can stimulate a national economy in the short term. During slack economic times, such as the Great Depression of the 1930s, military spending and war mobilization can increase capacity utilization, reduce unemployment, and generally induce patriotic citizens to work harder for less compensation (very Conservative!).
War also sometimes clears away outdated infrastructure (missile stocks etc) and allows economy-wide rebuilding, generating long-term benefits (albeit at short-term costs). For example, after being set back by the two World Wars, French production grew faster after 1950 than before 1914.