distance and the presence of two wide oceans helped to avoid aerial bombing and invasions.
Explanation:
The two main theaters of war (Europe and the Pacific) during WWI and WWII where separated from the US and Canada by the Atlantic Ocean and, basically, almost the entire Pacific Ocean. Due to the technical capabilities of that time, aerial bombing was impossible through these huge distances and an invasion (landings, establishment of bridgeheads and logistic network to sustain them) was basically impossible (considering that the D-day landings in Normandy or the invasions of the various atolls/Islands in the Pacific were already considered prodigious accomplishments).
