Complex sentence: The battle started when Companies F and I, 21st Infantry were ambushed by 1000 Filipinos, where there was terrific fighting, but the Americans running low on ammunition had to break through the enemy's flank.
Split into simpler sentences: The battle started when Companies F and I, 21st Infantry were ambushed by 1000 Filipinos. There was terrific fighting, but the Americans running low on ammunition had to break through the enemy's flank.