In the spring of Russia’s war on Ukraine, Washington DC seems haunted by the ghosts of history. The US Congress has passed the Ukraine Democracy Defense Lend-Lease Act of 2022 to expedite aid to Ukraine – just as Franklin D Roosevelt did, under the Lend-Lease Act, to the British empire, China and Greece in March 1941.

The sums of money being contemplated in Washington are enormous – a total of $47bn, the equivalent of one third of Ukraine’s prewar GDP. If it is approved by Congress, on top of other western aid, it will mean that we are financing nothing less than a total war.

Lend-Lease was a wartime intervention. The vast majority of the goods delivered were armaments. Monty’s army in the north African desert fought with Lend-Lease Sherman tanks. After 1942, the great Soviet counter-offensives were carried by Lend-Lease trucks.

What made this so extraordinary is that at the moment the Lend-Lease programme was launched in March 1941, the US was not in the war. Lend-Lease was the decisive moment in which the US, while not a combatant, abandoned neutrality. It forced jurists to come up with a new term to describe a stance of “non-belligerence”. In broader terms it marked the emergence of the United States as the hegemon that, for better and for worse, it remains today.

However, history is complex – scratch the surface and the ambiguities multiply. What does invoking Lend-Lease really imply for the direction of US policy?

Presumably, the narrative is sustained by the promise that a good war fought against an evil regime will be won through the generous sponsorship of the United States. But to complete that narrative arc you have to keep winding the clock forward from Lend-Lease in March to the Atlantic charter in August 1941 and, by December, to Pearl Harbor and the US entry into the war. Providing aid to both China and the British empire, Lend-Lease was a crucial step in turning what was originally a separate Japanese war on China and a German war in Europe into a world war.

If the US Congress is now launching a new Lend-Lease programme, the question of whether escalation is part of the plan must come into consideration.

Both friends and critics of FDR have always insisted that provoking a war with Nazi Germany was the hidden agenda of Lend-Lease. Most historians today would argue that the president’s intentions were more uncertain. Even after Pearl Harbor it was not obvious that Roosevelt could find a majority to declare war on Germany. As Brendan Simms and Charlie Laderman show in their book Hitler’s American Gamble, an extraordinary reconstruction of the fateful week following Pearl Harbor, the immediate reaction to the Japanese attack was to suspend Lend-Lease shipments; London and Moscow were horrified. It was not FDR but Hitler who saved the alliance by declaring war on the United States on the afternoon of 11 December.

Then, as now, it was our antagonists who were left with the choice of whether to escalate from economic to military confrontation. Then, as now, the motives of those antagonists are obscure.

After the announcement of central bank sanctions on 28 February, Putin rattled his nuclear sabre. If Biden signs a giant Lend-Lease-style aid package into law, who can tell how the Russian president will react? Further questions arise: will Ukraine be given weapons only to expel Putin’s army? Or will we equip Kyiv to strike at Russia itself?

In 1941, the main Anglo-American vision was to mount an unprecedented strategic bombing campaign to lay waste to Germany’s cities and “dehouse” its population. With conventional bombs that was a real slog. But part of the quid pro quo for the Anglo-American partnership was the Tizard mission, through which British know-how, including atomic bomb development, was transferred to the US. Behind the sugar-coated narrative of a good war won by the arsenal of democracy lurks the unleashing of an apocalyptic world war.

This was the nightmare that haunted Roosevelt’s opponents in America in 1941. They bemoaned the US being dragged into a second terrible conflict and the militarisation of the world order. And this was not a marginal point of view. Whereas the 2022 version of the Lend-Lease Act passed the Senate unanimously, in 1941 a third of the Senate voted against it.

Roosevelt knew that the American public was not ready for war. And he hoped that the Lend-Lease Act would allow him to avoid calling for it. This was the sentiment that Churchill played into when he appealed to the US in February 1941 not to enter the war, but to give Britain and its empire the tools “and we will finish the job”. But the very generosity and scale of Lend-Lease, and the commitment that implied, brought into stark relief the fact that the US was paying for others to fight the battle on its behalf.

That is precisely its position today. The US and its allies are for very good reasons choosing to back one side in a fight in which they will not directly engage. We do so like FDR, with one eye to the heroic resistance of those holding out against attack and with another eye to the geopolitical balance. If Russia has chosen to smash itself on the rock of Ukraine, if Ukraine is willing to fight, so be it.

If that is the plan and Putin allows us to stick to it, it certainly has logic on its side. It is a calculation so cold-blooded that it is little wonder that we want to dress it up in half-remembered histories of the second world war, in which the happy ending is assumed without the necessary sacrifices ever being spelled out.

• Adam Tooze is a professor of history at Columbia University