Sides...

I was looking through one of my history books for a suitable topic for today's random history when I came across something interesting.

August 23, 1914:
Japan declares war on Germany
{Don't Know Much About History by Kenneth C. Davis}

Most people know very little about World War I {myself included} so when I saw this little tidbit, my curiosity was spurred.
Nearly everyone knows that Japan joined forces with Hitler's Germany during World War II and their attack on Pearl Harbor in Hawaii gave the US a valid reason to join in the war.

Turns out that in the early 1900's, Japan was allied with the United Kingdom and wanted to expand into China's mainlands and surrounding islands. Japan told their allies that they would assist against Germany as long as they could take control of Germany's Chinese territories.

Since the United States did not want to get involved in World War I, the United Kingdom heavily relied on help from their other allies. The Japanese Imperial Navy was essentially given free rein in the far east, where they quickly took control of the German territories in China.
While the United Kingdom was engrossed in the war, Japan attempted to take further control of the Chinese with their Twenty-One Demands of the president {which would have turned China into a Japanese protectorate}.
The United States {among other countries} condemned Japan and the Japanese later withdrew their demands.
Overall, Japan helped the United Kingdom quite a bit during World War I mainly with exports of supplies and transporting/escorting troops.

So why did Japan change sides when World War II came around?

One word.
China. 

Japan is an island with only so much room and natural resources. Japan had become more militarized since World War I and were actively trying to take control of China's mainlands.
Once again the United States {among other countries} condemned Japan and stopped exports to the island country.
Japan was hoping that by attacking Pearl Harbor {among other places held by the British, Chinese and Dutch} the United States would want to sign a treaty to prevent more attacks and withdraw the embargoes against Japan.

Boy were they wrong and the rest is history. :)