Well Turkey had a complicated position in WWII and at different points in the war had different relationships with the major powers. But generally leaned toward the Allies.

In the fall and summer of 1939 there was a series of complicated negotiations with Germany, The Soviet Union, and Britain and France with both trying to get Turkey to agree to an alliance (remember at this time Germany and the Soviet Union had just carved up Poland and were both pushing for Turkey to not ally with the Western Powers). But Turkey did sign an alliance with the Western Powers for a few reasons. One was it had previously relied on the support of Soviet Union against Germany and didn't like them getting cozy. Another was that France had just ceded part of their mandate of Syria to them, so there was still goodwill there. So for the time being Turkey stopped shipping chrome to Germany, but after the fall of France this would change.

After the fall of France and before the invasion of the Soviet Union Germany sought and received assurances from Turkey that they would remain neutral in the upcoming war against the Soviet Union and began trading much needed chrome again. Britain wanted Turkey to join the war against Germany but they were afraid of the German's military might (and their border with the Soviet Union) as Germany was moving into the Balkans to help Italy's stalled campaign there, and they also wanted to potentially gain pieces of Greek territory that Britain certainly wouldn't help them take. So Turkey said they wouldn't join any front against Germany unless Germany attacked first.

In 1943 during the planning for operation Overlord, the British tried again to get Turkey to join a front against Germany, but failed. Churchill reportedly gave up on having Turkey enter the war by early 1944. However in August 1944 Turkey broke relations with Germany after the tide started to turn and they were starting to get driven out of the Balkans. Then in January 1945 Turkey broke relations with Japan at America's urging and opened the Bosphorus Straits to allied shipping.

But the final reason that Turkey declared war on Germany and Japan in February of 1945 is that at the Yalta conference the Big Three had decided that to join the United Nations you had to have declared war on Germany and Japan by a certain deadline. After Turkey declared war several other Latin American countries that had previously been neutral also declared war on Germany and Japan as well.

Source: A World at Arms: A Global History of World War II by Gerhard L. Weinberg.

As a point of clarification: Turkey did join the allies in February, 1945.