WW2 ended the Great Depression almost immediately, at least according to Alan Brinkley's The End of Reform.

But these positions aren't opposites: the New Deal started from high spending and inflationism in order to try and undo the huge deflation of the 1929 Crash; WW2 was like that on steroids - at times government spending reached approximately 50% of GDP. As the Brinkley book details, Keynesianism had been working its' way into the mindset of the American government at the end of the 30s and high spending and demand boost came to be seen as a legitimate policy instrument rather than a short-term expedient. The book details various plans for demand management that were planned during the war for the post-war period though most of them would never have passed Congress; the one that did was the GI Bill.

Also (and I can't recall the reference for this) WW2 was seen as a demonstration of the Keynesian thesis: massive government spending led to massive growth in the economy.

I think we have to first clean up your timeline.

The Great Depression starts around 1930. The Stock Market Crash is actually not necessarily the start. FDR takes over in 1933. The New Deal has two phases, the first new 'First New Deal' (1933–1934) and the 'Second New Deal (1935-1938). The US starts lend-lease in 1941 until 1945.

First we have to question the 'success' of FDRs policies. In 1933 FDR took the US of the gold standard and the US saw the fasted amount of industrial production growth in its hole history, but only for a few month. From that point on growth returned. Then the NRIA passes and recovery slows down, in the second new deal other policies pass that further slow down recovery. Its not until right before the war, some argue after it (because ration and war material production can not easily compared to peace time economy), that the economy fully recovers.

Everywhere outside of the US, the Great Depression was over much earlier. The US was by far one of the worse performers economically. So calling the New Deal a success in that sense is quite simply wrong. People can of course like it for other reasons, such a social policy or infrastructure, but from a purely economic growth point of view it was not.

See this for industrial production: https://uneasymoney.files.wordpress.com/2011/09/glasner-ind-prod.jpg

For US GDP growth see: https://en.wikipedia.org/wiki/Great_Depression_in_the_United_States#/media/File:US_GDP_10-60.jpg

The War really comes in right at the point when the economy has recovered to the point it was 10 years ago. The War certainly boosted output, but I don't think we can say the war 'fixed' anything.

After the War you have a different president, the Depression is in the past. People had made cutbacks, but not because of the impersonal economic forces, but rather to help the country with war. The US was the major exporter for the rest of the world and everybody had to rebuild. As one could expect, in this situation growth was quite strong from then on.

I very much want to stress that the monetary change was the most important thing. All the others were economically far less important. Once you fix the monetary system the economy can adjust. FDR delayed the process with high price policy (and others), but the major problem was the price and wages have to adjust.

Sources:

• A Monetary History of the United States, 1867-1960 - Friedman and Schwartz (a classic that laid down the monetary policy argument)

• What Ended the Great Depression? by Christina Romer

• The Midas Paradox: Financial Markets, Government Policy Shocks, and the Great Depression by Sumner (this book is not yet out, but the Profs that wrote it has many articles and lectures on the subject, Im sourcing the book because it will compile all this, see this video for example https://vimeo.com/11700175)

• Freedom from Fear: The American People in Depression and War, 1929-1945