On October 29, 1914, the Turkish warships Yavuz Sultan Selim and Medilli, which until a short time before had been the German battle cruiser Goeben and light cruiser Breslau, attacked Sevastopol, Odessa. and other Russian Black Sea ports. The Ottoman Empire had been neutral; but then the Goeben and Breslau, fleeing British vessels in the Mediterranean, sailed into Constantinople and it was announced that Germany had transferred them to Turkey. The German crews put on Fezzes and became part of the Ottoman fleet.
The October 29 raid on Sevastopol was carried out without approval of the Ottoman Cabinet, though likely with the knowledge of the War Minister, Enver Pasha. On October 31 Enver ordered men to report for conscription and formally joined the Central Powers; on November 2, Russia declared war, and the British followed three days later.
I previously ran this clip on the Goeben/Yavuz:
And I also ran across this photo from November 11, 1914, showing the Ottoman Sheikhulislam (the chief religious figure) announcing the war and proclaiming a Jihad. The British worried that this might lead to a rising of Muslims in India, but it never did; the Sultan's claim to be Caliph of Islam was not widely acknowledged outside the Ottoman territories themselves.
The Ottoman decision to go to waron the side of the Central Powers was, of course, fatal to the Empire.