Initially, this proposal failed, but after Germany and the Soviet Union invaded Poland in September, Congress passed the Neutrality Act of 1939 ending the munitions embargo on a "cash and carry" basis.

Hampered by public opinion and the Neutrality Acts, which forbade arms sales on credit or the lending of money to belligerent nations, Roosevelt eventually came up with the idea of "lend–lease".

The British shared technology included the cavity magnetron (key technology at the time for highly effective radar; the American historian James Phinney Baxter III later called "the most valuable cargo ever brought to our shores"),[13][14] the design for the VT fuze, details of Frank Whittle's jet engine and the Frisch–Peierls memorandum describing the feasibility of an atomic bomb.

[15] Though these may be considered the most significant, many other items were also transported, including designs for rockets, superchargers, gyroscopic gunsights, submarine detection devices, self-sealing fuel tanks and plastic explosives.

[nb 2][16] In his December 29, 1940 Fireside Chat radio broadcast, President Roosevelt proclaimed the United States would be the "Arsenal of Democracy" and proposed selling munitions to Britain and Canada.

In time, opinion shifted as increasing numbers of Americans began to consider the advantage of funding the British war against Germany, while staying free of the hostilities themselves.

[17] Propaganda showing the devastation of British cities during The Blitz, as well as popular depictions of Germans as savage also rallied public opinion to the Allies, especially after Germany conquered France.

"[18] Opposition to the Lend-Lease bill was strongest among isolationist Republicans in Congress, who feared the measure would be "the longest single step this nation has yet taken toward direct involvement in the war abroad".

[22] After the attack on Pearl Harbor and the United States entering the war in December 1941, foreign policy was rarely discussed by Congress, and there was very little demand to cut Lend-Lease spending.

Roosevelt's Soviet Protocol Committee was dominated by Harry Hopkins and General John York, who were totally sympathetic to the provision of "unconditional aid".

[36] WWII was the first major war in which whole formations were routinely motorized; soldiers were supported with large numbers of all kinds of vehicles, not just for direct combat roles, but for transport and logistics as well.

[clarification needed][44] According to the Russian historian Boris Vadimovich Sokolov, Lend-Lease had a crucial role in winning the war: On the whole the following conclusion can be drawn: that without these Western shipments under Lend-Lease the Soviet Union not only would not have been able to win the Great Patriotic War, it would not have been able even to oppose the German invaders, since it could not itself produce sufficient quantities of arms and military equipment or adequate supplies of fuel and ammunition.

No one ever discussed this subject officially, and I don't think Stalin left any written evidence of his opinion, but I will state here that several times in conversations with me he noted that these were the actual circumstances.

[45]In a confidential interview with the wartime correspondent Konstantin Simonov, the Soviet Marshal Georgy Zhukov is quoted as saying: Today [1963] some say the Allies didn't really help us ...

Lend-Lease aid did not arrive in sufficient quantities to make the difference between defeat and victory in 1941–1942; that achievement must be attributed solely to the Soviet people and to the iron nerve of Stalin, Zhukov, Shaposhnikov, Vasilevsky, and their subordinates.

Perhaps most directly, without Lend-Lease trucks, rail engines, and railroad cars, every Soviet offensive would have stalled at an earlier stage, outrunning its logistical tail in a matter of days.

In turn, this would have allowed the German commanders to escape at least some encirclements, while forcing the Red Army to prepare and conduct many more deliberate penetration attacks in order to advance the same distance.

Left to their own devices, Stalin and his commanders might have taken twelve to eighteen months longer to finish off the Wehrmacht; the ultimate result would probably have been the same, except that Soviet soldiers could have waded at France's Atlantic beaches.

The Lend-Lease agreements with 30 countries provided for repayment not in terms of money or returned goods, but in "joint action directed towards the creation of a liberalized international economic order in the postwar world."

This was the reason that the supply of aircraft, primarily fighters and bombers, became the main topic in the negotiations between the top leadership of the USSR, Great Britain and the United States.

But even those that could be called obsolete (the English Hurricane and the American Tomahawk) were more advanced and superior in most characteristics than the I-153 and I-16 aircraft that made up the basis of Soviet fighter aviation in the most difficult first months of the war.

[65][66] On 12 July 1941, within weeks of the German invasion of the USSR, the Anglo-Soviet Agreement was signed and the first British aid convoy set off along the dangerous Arctic Sea route to Murmansk, arriving in September.

[78] Reciprocal contributions included the Austin K2/Y military ambulance, British aviation spark plugs used in B-17 Flying Fortresses,[78] Canadian-made Fairmile launches used in anti-submarine warfare, Mosquito photo-reconnaissance aircraft, and Indian petroleum products.

It is an indication of the extent to which the British have been able to pool their resources with ours so that the needed weapon may be in the hands of that soldier—whatever may be his nationality—who can at the proper moment use it most effectively to defeat our common enemies.

Our Fortresses and Liberators take off from huge air bases built, equipped and serviced under reverse lend-lease at a cost to them of hundreds of millions of dollars.

The cooperation that was built up with Canada during the war was an amalgam compounded of diverse elements of which the air and land routes to Alaska, the Canol project, and the CRYSTAL and CRIMSON activities were the most costly in point of effort and funds expended.

Some idea of the scope of economic collaboration can be had from the fact that from the beginning of 1942 through 1945 Canada, on her part, furnished the United States with $1,000,000,000 to $1,250,000,000 in defense materials and services.

Although most of the actual construction of joint defense facilities, except the Alaska Highway and the Canol project, had been carried out by Canada, most of the original cost was borne by the United States.

The dispute remained unresolved until 1972, when the U.S. accepted an offer from the USSR to repay $722 million linked to grain shipments from the U.S., representing 25% of the initial debt with inflation taken into account, with the remainder being written off.

[87] In June 1942, SS Port Nicholson was sunk en route from Halifax to New York, allegedly with Soviet platinum, gold, and diamonds aboard; the wreck was discovered in 2008.