Starting in 1939, any nation fighting in World War II could purchase goods and arms in the United States under the "Lend-Lease" program or act.
Countries such as Britain, China, the Soviet Union, Brazil among others received weapons under this program.
According to the Neutrality Act the U.S can not receive any compensation for the arms. Instead, they covertly were present in the war and defend its own interests by giving arms and other goods to the countries they specifically chose.