During the 1920s and 1930s General Motors bought out the bus company Yellow Coach[?], helped create Greyhound bus lines, replaced intercity train transport with buses, and established subsidiary companies to buy out tram (streetcar) companies and replace the trams by buses. General Motors bought the train engine maker Winton Engine[?] in 1930, and shifted production from electric to diesel engines. This encouraged the use of fossil fuels and made it difficult to change social policies of energy generation[?].
It a common belief popularized by Bradford Snell[?] that General Motors was convicted of conspiracy in the 1950s in their program to buy up and destroy electric urban trolley systems so that urban transit would be forced to rely on GMC buses and that this is the principal reason that modern-day trolley systems are rare in the United States today. This belief has been questioned by Sy Adler[?] who points out that, among other things, that General Motors was not convicted of buying up urban trolly systems but rather merely of forcing bus companies owned by General Motors to use General Motors buses and that trolley ridership peaked in the 1920 before GM's actions.
General Motors supported opposing sides during World War II. According to a report presented to the United States Senate in 1974, during the 1920s and 1930s, General Motors (and Ford and Chrysler) expanded to many countries in Europe, including Germany. They continued to supply trucks to both the US military and the German military. The report claims that General Motors and Ford subsidiaries built nearly 90 percent of the armored "mule" 3-ton half-trucks and more than 70 percent of the Reich's medium and heavy-duty trucks. These vehicles, according to American intelligence reports, served as "the backbone of the German Army transportation system".
The chairman of General Motors at the time, Alfred P. Sloan, allegedly defended this support of the German government, because GM's operations in Germany at that time were "highly profitable".
At one point it was the largest corporation in the United States ever, in terms of its revenues as a percent of GDP.