Complex sentence: In this endeavour, SPD leftists sought an alliance with the Supreme Command and had the army and Freikorps (nationalist militias) and quelled the so-called Spartacist uprising by force.
Split into simpler sentences: In this endeavour, SPD leftists sought an alliance with the Supreme Command. The army and Freikorps (nationalist militias) quelled the so-called Spartacist uprising by force.