Complex sentence: It involved 15,000 American, Afghan, British, Canadian, Danish, and Estonian troops; constituting the largest joint operation of the War in Afghanistan up to that point, aimed at removing the Taliban from Marja, eliminating the last Taliban stronghold in central Helmand Province.
Split into simpler sentences: It involved 15,000 Afghan, American, British, Canadian, Danish, and Estonian troops; constituting the largest joint operation of the War in Afghanistan up to that point. The purpose of which, to remove the Taliban from Marja, eliminating the last Taliban stronghold in central Helmand Province.