Here's just one example:
Because the axis powers (Germany, Italy, and Japan) had their eyes on conquering the world. And they were acting upon those goals. They invaded France, Poland, Western Russia, Manchuria, the Philippines, annexed Austria and Hungry, island hopping across the Pacific, and bombed Pearl Harbor.
Negotiations had been tried (N. Chamberlain) and despite agreements, their advancement did not cease.
It was only by use of military force that they were stopped.