Complex sentence: The Allies had failed to cross the Rhine and the river remained a barrier to their advance into Germany until offensives at Remagen, Oppenheim, Rees and Wesel in March 1945.
Split into simpler sentences: The Allies had failed to cross the Rhine. The river remained a barrier to their advance into Germany until offensives at Remagen, Oppenheim, Rees and Wesel in March 1945.