How did the actions of Japan, Germany, and Italy in the first half of the 1930s serve as warning signs of an impending war?

The seeds for the start of World War II were sown by the eventual Axis powers of Japan, Italy, and Germany in the first half of the 1930s. The Western powers, still wearied from World War I, hoped that appeasement would keep the peace, but their reaction only emboldened the dictators into believing that their actions would have no consequences. This led to further aggressiveness later in the decade until the West had no choice but to go to war to stop the Axis powers.
The first action that presaged world war was Japan's invasion of Manchuria in 1931. The Japanese coveted resources on the Chinese mainland and used the pretext of an explosion that destroyed a railway owned by the Japanese to invade the region. The Chinese army was ill-prepared to stop the Japanese, who quickly took control and turned Manchuria into the puppet state of Manchukuo. The West's response was simply not to recognize the new state or any treaties that it made.
Next to test the Western powers was Italy, which decided to resume hostilities with Ethiopia in 1935. The Italians had tried to invade Ethiopia in 1896 but were humiliated in battle. They were determined to win this time and overwhelmed the Ethiopians with modern weaponry. The League of Nations, created after World War I to try to resolve conflicts without warfare, voted to impose sanctions upon Italy for its aggression, but the sanctions were weak, and member nations were lax in enforcing them. The Italians ignored the sanctions and completed their takeover.
Finally, it was Germany's turn to take action. After Adolf Hitler took power in 1933, the Germans had steadily increased its resistance to the restrictions imposed upon it by the Treaty of Versailles that had ended WWI. Finally, in March 1936, the Germans marched into the Rhineland region—an egregious violation of the Treaty, which provided for the region to be a demilitarized buffer zone between Germany and France. If the West had called Hitler's bluff and met might with might, he would have been forced to retreat, as the German army was not yet ready to fight a major conflict with France and England. Instead, the West protested but did nothing else. Hitler learned the same lesson the Japanese and Italians had learned earlier—he could act with impunity with little resistance from the West.
https://history.state.gov/milestones/1921-1936/mukden-incident

https://www.britannica.com/event/Italo-Ethiopian-War-1935-1936

https://www.nationalarchives.gov.uk/education/resources/german-occupation/