Complex sentence: Within a few days, the U.S. War Department was able to arrange for the strategically located fort to be reinforced with two Massachusetts volunteer militia regiments, which were trained and ready to respond to a call for federal troops.
Split into simpler sentences: Within a few days, the U.S. War Department was able to arrange for the strategically located fort to be reinforced with two Massachusetts volunteer militia regiments. The regiments were drilled and ready to respond to a call for federal troops.