In addition to the already-stated causes of government issuing currency necessary to meet war spending and the fact that war spending produces destruction of economic capability rather than development, wars tend to introduce trade barriers and divert resources away from productive tasks. Whether barriers are legal (tariffs, embargoes), or simply higher premiums due to increased risk, less trade happens, which raises prices (inflation). Economists also really hate number-go-down even when down is good, so policy is oriented towards making sure deflation never gets a chance in the interwar development periods.