To answer this, we have to ask ourselves a few questions:

1). Who were the main powers in the Pacific? 2). What caused Japanese aggression? 3). What caused the Western passivity?

We will see that the last two questions have largely the same answer.

Japan's rise was truly recognized in the aftermath of the Russo-Japanese War, in which the Japanese handily defeated the Russians at sea and land (note that in a prolonged engagement, Japan would have suffered economically). Japanese aggression, though persistent throughout the 1920's, really exploded in 1931 with the invasion of Chinese Manchuria. At this time, the other major powers on the Pacific were the British, the Americans, the French, and the Soviets. Historically, the British had used the Japanese as a foil to other foreign powers becoming too dominant in Asia (see the Anglo Japanese Alliance of 1902), and the United States had, since the late 1800s, advocated that all foreign powers limit direct influence in Asia. Essentially, they Western Powers regarded Japan in the early 20th century as a powerful nation, but one that could be controlled.

What contributed to Japanese aggressive actions? Well, the short answer is: economic depression. The Great Depression sent shock waves around the world, and Japan was highly dependent on American money and trade. When much of that was withdrawn to deal with domestic issues, Japan was faced with a crisis, her economy shrinking by 8% from 1929 to 1931. Coupled with a long standing desire for international prestige (exacerbated by feeling cheated out of certain concessions by the West after the First Sino Japanese War), Japanese nationalists easily took power. Their answer to the economic crisis: conquest while the other powers are unwilling to intervene. As things got worse for the western powers as the 1930s progressed, Japan went on an all out offensive in 1936.

The Great Depression essentially crippled the Americans, British, and French, and contributed hugely to Hitler's ascension, further diverting attention from Japanese aggression. In Britain, by 1931 exports had halved, and some areas reported unemployment rates at 70%. Even France's relatively high self sufficiency was unable to stave off the effects of the Depression, and extreme right parties took power. In 1931, with the exception of the Soviet Union, we see the major Pacific powers unable to respond to Japanese aggression against China. By 1936, only the British and Americans took any measures to rein in the Japanese.

By 1936, Hitler was taking Germany on its road to war, and the French and Soviets were bracing themselves as best as they could. The Soviets took some measures to protect their eastern territories, but following Stalin's purges, was in no shape to prepare for conflict on two sides. With the French metropol directly in danger, Asia was a sideshow for France.

Also, 1931 was the first true test for the League of Nations. Despite being the foremost members, neither France nor Britain wished to take action against Japan, and were more interested in managing their empires. This in action was repeated in 1936, when the League simply issued a reprimand against Japan, prompting the Japanese to leave the League.

The British did take measures (as much as their liberal government and economic constraints would allow) to beef up their Asian holdings leading to 1941, but again, restraining Germany was its primary objective. To protect the Singaporean base, the headquarters of their East Asian divisions, the British could only afford to send two ships, which were quickly sunk by the Japanese. It was not until the United States tapped its full industrial potential (corresponding chronologically to stalling German advances) that the British had the resources and attention to push back against Japan.

The United States had actually applied every kind of non violent pressure on Japan to hold it back. It cut trade and imports to Japan (previously supplying 60% of Japan's oil), it froze Japanese bank accounts, and it even supplied weapons to the Chinese to keep them fighting. The United States had, since 1910, had plans to fight Japan (War Plan Orange), but politically in the late 1930s, Germany seemed to be the greater threat.

Essentially, the Japanese were not stopped earlier because they began their aggression when the Western powers were distracted economically, and later expanded their interests when the Westerners were too focused with Germany to really care about Japanese conquests in China.