Complex sentence: By the 1920s and 1930s, large mail-order houses such as Sears, Roebuck & Co. and Montgomery Ward provided serious competition, so the department stores came to rely even more on salesmanship as well as close integration with the community.
Split into simpler sentences: By the 1920s and 1930s, large mail-order houses such as Sears, Roebuck & Co. and Montgomery Ward provided serious competition. In response the local department stores came to rely even more on salesmanship as well as close integration with the community.