This is a tough question that American policymakers have tried to tackle since the start of the Second World War. Franklin Roosevelt, who was never an isolationist himself, believed that Woodrow Wilson's failure to bring the United States into the League of Nations had been a cause of global instability, and fixing this mistake became a passion for him over the course of the war (political opponents criticized him after his death for giving up too much to the Soviet Union in order to ensure the creation of the United Nations). Other policymakers focused more on the "Lesson of Munich," which warns that Chamberlain's negotiations with Hitler that culminated in the German annexation of the Czechoslovak Sudeten region (this itself was a complex issue... France and Britain had sold out their ally Czechoslovakia, but then no one wanted to go to war to prevent Germany from annexation a border region that was ethnically German). Lyndon Johnson, for example, cited Munich--but not specifically American isolationism--in a speech justifying the escalation of the War in Vietnam. But these concepts are linked. The idea is that the United States is capable of preventing a global war by projecting its power across the globe and that inaction prior to WW2 proves this.

The problem with these conclusions is that they always rely on counterfactuals in which you can change any idea that you like without having to explain how these changes occurred. For example, you could say "what if the United States had decided to declare war on Germany at the start of WW2?" But if you want a serious answer, you have to create new counterfactuals to explain this change, and following cause and effect to its fullest, you end up with an entirely new and made up world. This isn't to say that counterfactuals are never useful--indeed, if policymakers can find a way that the United States could have prevented the Second World War, that would be quite useful--but I think they sometimes are used to obfuscate the nuance of what really happened, with "American isolationism as a cause of WW2" being a perfect example. First lets look at some of the most popular "what ifs" and then I'll explain what I mean.

What if the United States had joined the League of Nations in 1920?

This is the most popular of all the what ifs, and it even influenced FDR. Here we can imagine an incredibly strong League led by the United States that would have put a stop to Hitler's remilitarization of Germany before it started. There never is a WW2 because aggressive international diplomacy, backed by the power of American military potential, would prevent any single Great Power from starting a power again.

What if the United States had not responded to Hitler's rise to power by promoting an isolationism, most notably through the Neutrality Acts, that made clear to Europe that it would not intervene to help stop Germany's rearmament?

FDR, president at the time, opposed the Neutrality Acts and isolationism generally. In 1937, just a year before Munich, he delivered a speech in Chicago, a center of isolationist thought, warning that the United States needed to help "quarantine" aggressive powers. His speech failed to sway public opinion, however. But here we find a clearly defined what if... what if FDR had swayed public opinion? Could the United States have worked with the other powers that feared German aggression--Great Britain, France, and the Soviet Union, not to mention other middle powers like Czechoslovakia that bordered Germany--to stop the war before it began?

What if the United States had joined the war right away?

Another what if that does not prevent the war, but perhaps brings it to an early end. After all, war was declared in 1939, but fighting between the Great Powers did not begin until 1940.

You can imagine a positive outcome to any of these counterfactuals. You can also imagine that the war rages on anyway. For example, the League of Nations proved impotent without American involvement, and there is no reason to think that, had America joined the organization at its foundation, that it would have remained an international force more than a decade and a half later. If American sentiment turned against interventionism anyway, then the League would have remained impotent in this imagined timeline as well. As for point three, you could imagine that intervention in 1939, when Germany and the Soviet Union remained allied, could have proved disastrous as well. Breaking that alliance might have been more important than ending the war... or maybe not. Who knows.

This brings me to point #2, which I think is the meat of your question: What if the United States had shirked isolationism in favor of interventionism prior to the Second World War? But here I think we find a comforting American illusion, originally dispelled by Arnold Offner in American Appeasement: United States Foreign Policy and Germany, 1933-1938 (1969), that the United States would have necessarily attempted to stop German expansion if the American people had let FDR intervene. In fact (and this is the area of my research), we find in the diplomatic record American policymakers and diplomats who strongly support the efforts to cede the Sudeten region to Germany. Ambassador to France William Bullitt warns against Czech "chauvinism" and advised FDR that while he will be remembered for "selling out" a smaller nation, that he should not "hesitate to take that brick to [his] head." FDR agreed with Bullitt, and even went so far as to offer Hitler--but not the Czechoslovaks--to mediate a meeting if only the Fuhrer would give peace one more chance! About the same time, Chamberlain wrote to FDR guaranteeing that he could appease Hitler, and FDR retracted his offer. The Czechoslovak foreign ministry found out about the offer and quite literally begged FDR to host the proposed meeting, but he ignored them. In the end, FDR had nearly shirked American isolationism, an act that would have cost him dearly in the eyes of the public, in order to ensure the transfer of the Sudeten region to Hitler!

Perhaps in another world, the American public rallied around FDR after his 1937 quarantine speech and he used this newfound support to end German expansion. As it really happened, FDR supported the German annexation of the Sudeten region in the 1938, which analysts have ever since regarded as a decisive step towards World War II. The reality is that FDR's perceptions of the world mirrored those of his allies in Western Europe, and while American power could have certainly proved decisive if it had been wielded differently, it's not clear that FDR would have prevented the war even if the American public have supported such intervention. It's possible that we would blame FDR for the appeasement of Hitler, rather than Chamberlain, if FDR had intervened in European politics before the war.

The answer is pretty much no for WWI.

I'm trying to think of a way that the US policies could have been possibly involved in the start of WWI and I'm just not seeing any. Perhaps an alternative history exists where the US got super involved in Balkan politics but that would be a significant switch from reality. The isolationism existed, but it would be a stretch to say that it was an essential precursor.