It is lend-lease act and it was fundamental to turning the tide of the war as Great Britain and Russia were broke and had no reserves to pay for American armaments or goods. American public opinion opposed involvement in the war and that resulted in the passage of the neutrality acts. Neutrality acts prevented us from lending to belligerents or becoming financially involved because that was seen as the reason US forces were drawn into WW1. The Neutrality Acts were modified in 1939 to allow cash purchases of goods but Britain soon notified Roosevelt that they were broke because of conversion of industry to war production and away from trade goods. Lend Lease was the answer to this problem and effectively ended the neutrality acts and began US entry into the war by escorting convoys delivering lend lease goods. Repayment was supposed to be in kind and Russia was the last that I know of that repaid their debt in 1990.