Complex sentence: It was based on the American Caterpillar 60 tractor and was mostly produced for the Red Army, which used them extensively to haul artillery, as the tractor was heavily built and had a powerful engine.
Split into simpler sentences: It was based on the American Caterpillar 60 tractor. The tractor was mostly produced for the Red Army, which used them extensively to haul artillery, as the tractor was heavily built and had a powerful engine.