The US has revived its historic Lend-Lease policy to help Ukraine. This was a World War II era policy in support of the enemies of Nazi Germany.
When the UK and France declared war on Nazi Germany after its invasion of Poland, the US initially remained neutral. After the fall of France, Britain remained the only major power resisting Germany. This changed only when Hitler attacked the Soviet Union in the summer of 1941. From 1939 to 1941, the Soviet Union had been an ally of Nazi Germany under the Molotov-Ribbentrop treaty and had supplied Germany with oil for its invasion of Denmark, Norway, France and Benelux.
After the fall of Dunkirk, Britain had been almost on its own (it was still supported by Australia, Canada, New Zealand and British colonies). It could buy supplies from the US but it had to pay in cash (i.e. silver) and transport the goods to Britain on its own ships. With the passage of Lend-Lease in March 1941, the US could supply arms, ammunition and other goods to Britain without requiring payment and it sent them to Britain on American ships.
After the German attack on the Soviet Union (Operation Barbarossa), the USSR was also supplied by the US with tanks, trucks, ammunition, food and many other materials for the war effort. These supplies were sent via Persia, Murmansk and the Far East. When Putins’s Russia today talks about the Soviet victory over Nazi Germany, it consistently remains silent about the US contribution to the Soviet war effort, without which the country may have collapsed.
Lend-Lease was controversial in the US. Isolationists and nazi sympathisers argued it would put the US on a slippery slope towards entering the war in Europe. However, the US still officially remained neutral and not at war with Germany and its allies. This only changed in December 1941 when Imperial Japan attacked Pearl Harbor. A few days after Pearl Harbor, Hitler declared war on the US in support of its ally in the Far East. Thus it was not Lend-Lease that got the US involved in World War II but the Japanese attack.
This holds a message for today. Countries do not automatically become parties in war if they support another country with supplies.
Wars of aggression are a violation of the UN Charter which guarantees the territorial integrity of all states and requires them to use peaceful means to settle any disputes. Every country has a right of self-defense. Under International law, countries worldwide have every right to support countries exercising their right of self-defence against aggressors and invaders. That is as true in 2022 as it was in 1939 or 1941.