Complex sentence: Upon assuming control of the SA in January 1931, Ernst Roehm attempted conciliation with Buch over SA-Uschla affairs but these attempts crashed upon the rock of the Stennes Affair.
Split into simpler sentences: Upon assuming control of the SA in January 1931, Ernst Roehm attempted a conciliation with Buch over SA-Uschla affairs. However, these conciliatory attempts crashed upon the rock of the Stennes Affair.