Complex sentence: After the war Blumentritt was called as a witness at the Nuremberg Trials, though he never testified, and then later helped in the rearmament of Germany during the Cold War and the development of the modern German army.
Split into simpler sentences: After the war was called as a witness at the Nuremberg Trials, though he never testified. Later, he helped in the rearmament of Germany during the Cold War and the development of the modern German army.