While it is well known that the Gedolei Yisroel disagreed with Rabbi J.B. Soloveitchik regarding his general approach to Orthodoxy, one point I think they all agreed with was that he was a godol b’kiruv.
The prewar and immediate postwar periods was a time that saw a terrible shedding of observant Jewry in the United States. Many previously nominally Orthodox congregations and adherents switched their affiliation to Conservative Judaism or other forms of non-observant Judaism. RJBS saw a need for a movement for people who weren’t ready or capable of being fully observant, and not wanting them to officially discard whatever amount of observance they still adhered to, effectively created an Orthodox-Lite movement that kept the core values of the Torah, even if it didn’t slam their affiliated members for the areas of Jewish law that they were non-observant of.
And, hence, we have what is known as Modern Orthodoxy.