Among the many unwanted gifts Donald Trump has given international law as of yet, this may very well prove to be the worst: the humanitarian reprisal. Forcible countermeasures, so-called reprisals, were standard practice in order to enforce violations of international obligations at least until World War I and continued to be used and accepted even in the inter-war period. Not infrequently, they led to wider military conflicts. Thus, under the post-1945 international legal order established by the UN Charter, reprisals do not constitute licit countermeasures and in fact are covered by the prohibition of the use of force in Article 2(4) of the UN Charter.