When war broke out in 1939 Kaser, despite being still relatively young, was exempted from military service on account of a serious heart defect. However, as the slaughter of war intensified such matters counted for less, and in November 1943 he was conscripted into the army. He was initially engaged in anti-aircraft warfare but was soon sent back to Münster where he worked as a clerk for the regimental administration ("Regimentsstab"). The army permitted him still to lecture at the university for two days each week. In April 1945 the Americans came. War ended a month later and Max Kaser became a prisoner of war. He was held in various American and French camps till his release, which followed in 1946. He returned to the university and resumed his teaching. The National Socialists had had their own views on Roman law. They found it "materialistic" and incompatible with their racist ideals, so they planned to replace the German legal code with their own People's Legal code which would (among other things) remove the influence of Roman Law over the German system. But now they were gone. Kaser could now be less constrained in his lecturing, his writing and on his networking. His reputation continued to grow. Offers of professorships came from Marburg (again) and Heidelberg (again). There were also offers from Graz (1951), Göttingen (1952) and Vienna (1959). Till 1959 he rejected all such offers.