Complex sentence: The Order was forced to become a branch of the Teutonic Knights in Prussia, making Samogitia, a strip of land that separated Livonia from Prussia, the main target of both orders.
Split into simpler sentences: The Order was forced to become a branch of the Teutonic Knights in Prussia. That meant that Samogitia, a strip of land that separated Livonia from Prussia, became the main target of both orders.