Isolationism contributed to the "march of aggression" because it allowed the aggressors to get what they wanted without having to worry about other countries interfering with them.
During this time, countries like Britain and, especially, the United States, were pursuing a policy of isolation. Therefore, they did not interfere when other countries like Germany and Japan started to act aggressively. When Germany took the Sudetenland and Czechoslovakia, neither country did anything. When Japan invaded China, neither country did anything.
When countries like the US and Britain did not oppose these acts of aggression, Japan and Germany continued to be aggressive, thus helping to bring on WWII.
Isolationism contributed to the aggressive actions that led to the start of World War II. There were various reasons why the United States, Great Britain, and France followed isolationist policies. These countries were in the middle of dealing with the Great Depression. They had to focus on solving the economic issues at home. There was also concern that trying to stop the aggressive countries could lead to war. None of the Allied countries were ready for possibly having another war so soon after World War I ended. There were people in the United States who felt we entered World War I so businesses could profit. These people didn’t want that to happen again.
As a result, when Japan invaded Manchuria in 1931 and China in 1937, there was no action taken. When Italy invaded Ethiopia in 1935, the invasion elicited no significant response from the Allies. When Germany annexed Austria in 1938, there was no response. Each time an aggressive action was met with little or no response, it sent an unintended message to the aggressive nations that it was acceptable to take land from other countries. When the Allies eventually did take action, it was too little, and it was too late. The path to World War II was already charted.
While the reasons for pursuing isolationist policies made sense, the results were disastrous.