/u/cabroney already explained that it was in the context of the Sino-Soviet split. Vietnam (seen by China as encouraged by the Soviets), after attacking Cambodia and stationing troops in Laos, were accused of regional hegemony right next to China. Deng, however, had never intended the war to last more than a couple of months at most. The reason for the short war was that "China needed a safe, reliable environment to undertake its Four Modernisations". A protracted war would slow down China's economic development. Furthermore, at the time, Deng thought that PLA spending was way too high. A side effect of the war was that because the PLA suffered heavy losses, this allowed Deng to declare that that the PLA needed to be reformed. This resulted cuts to military spending that went towards economic reform.