Complex sentence: Prior to TCA, no large national airline existed in Canada and with war looming and other nations (primarily the U.S.) experiencing major increases in the creation of passenger airlines, it was necessary to have a presence.
Split into simpler sentences: Prior to TCA, no large national airline existed in Canada. With war looming, and other nations (primarily the U.S.) experiencing major increases in the creation of passenger airlines, it was necessary to have a presence.