Technically they did intervene on the side of the Allies in February of 1945, but not to any serious effect. The leader of Turkey at the time met with Churchill and Roosevelt, but was very wary of entering a war for which they were ill-prepared. Their navy was tiny and their army was using outdated weaponry. The country was still quite unstable as well.

In exchange for intervention they also wanted a guarantee of American support if the USSR ever invaded the Turkish Straits and the Americans weren't willing to promise that.

See Deringil's Turkish Foreign Policy During the Second World War: An Active Neutrality if you're really interested!

"The Grand Delusion" by Gorodetsky both blows apart many Western-held myths about Barbarossa, and goes into much detail about manoeuvers in the Balkans/Black Sea region. The (very stupid) British intervention in Greece was largely defended by Churchill on the grounds of its influence on Turkey.