With the Allied evacuation of Gallipoli at the end of 1915, substantial Ottoman forces would be released to reinforce their other fronts. Realizing this, the Russians made a pre-emptive strike in eastern Anatolia with an army of 300,000; their first target being the formidably fortified city of Erzerum. After routing the Ottoman forces sent to confront them, the Russians subjected Erzerum to a massive bombardment leading to its evacuation on 16 February. The Russian armies then fanned out, with two detachments converging on Trebizond from the east and south. Meanwhile, the Russian navy landed reinforcements at Atina and captured the seaport of Riza on 17 March. In late March, Ottoman reinforcements finally arrived from Gallipoli and fought a determined, rearguard action against the Russians marching from Riza, but to no avail. With its protective forts destroyed by Russian bombardment, the Ottomans evacuated Trebizond on 18 April.