Complex sentence: The Army Group Vistula (also known as Army Group Weichsel) was formed in 1945 to protect Berlin from the advancing Soviet armies marching from the Vistula "(German: Weichsel)" river.
Split into simpler sentences: The Army Group Vistula (also known as Army Group Weichsel) was formed in 1945. It was formed to protect Berlin from the advancing Soviet armies advancing from the Vistula "(German: Weichsel)" river.