Short answers, pulled from the story.
The United States entered World War I in April 1917 without a single tank of its own. General John Pershing recognized the need for both light and heavy tanks after reviewing British and French operational theories.
A total of 49,234 M4 tanks were produced during World War II, making it one of the most used American tanks in history. Production began on the 5th of September 1941 after the T6 pilot model proved acceptable to U.S. Army officials.
On the 22nd of December 1941, the 192nd Tank Battalion became the first American unit to engage enemy armor in tank-to-tank combat against tanks from the Imperial Japanese Army 4th Tank Regiment. This engagement occurred during the Bataan peninsula campaign in December 1941.
A total of 2,222 M26 Pershing tanks were produced beginning in November 1944, though only 20 saw combat in Europe during World War II. The tank came into service in 1945 under the designation M26 heavy tank named after General John J. Pershing.
On the 8th of August 1950, the first M46 Pattons deployed to South Korea proving superior to much lighter North Korean T-34-85s encountered in relatively small numbers. By end of 1950, 200 M46 Patrons had been fielded forming about 15% of US tank strength in Korea.