Of course, the United States’ policies were not the major contributing factor to the outbreak of World War II. Instead, it was the actions of Germany in Europe and Japan in Asia that were the most important factors. However, it is possible to argue that the US could have prevented the war from happening (or at least made it much less likely) if the country had been more assertive and less isolationist.
We can say that WWII happened because the countries that were satisfied with the status quo after WWI (notably France, Britain, and the US) did not do enough to defend that status quo. These countries did not want to go to war again and so they let the countries that were dissatisfied (mainly what would become the Axis Powers) act aggressively. It is at least possible to argue that a firmer stance would have prevented this.
For example, then, the US could have pushed hard to keep Germany from breaking the terms of the Treaty of Versailles. It could have tried to intervene aggressively to stop Japan’s incursions into Manchuria and eventually into China. Instead, the US was content to remain relatively isolationist. By doing nothing, we can argue, the US allowed the dissatisfied countries to get stronger and stronger until they felt able to go to war.