N0m: the MO *are*’ the left-wing of the Orthodox.
Before Rav Moshe came to America, there was very little Orthodoxy in America other than MO. A sizable traditional Orthodox community in America, aside from MO, only occurred in the postwar period with the influx of European Jewry to American shores in the post-Holocaust period. The trickle started coming in the leadup to WWII as well as during the war period. But the large numbers came postwar.