Complex sentence: Delta grew through the addition of routes and the acquisition of other airlines; it replaced propeller planes with jets in the 1960s and entered international competition to Europe in the 1970s and across the Pacific in the 1980s.
Split into simpler sentences: Delta grew through the addition of routes and the acquisition of other airlines. They replaced propeller planes with jets in the 1960s and entered international competition to Europe in the 1970s and across the Pacific in the 1980s.