Complex sentence: One of the major challenges facing the D-Day landings was to secure a European port for the delivery of essential supplies, but instead of capturing an existing port, the Allies decided on an audacious plan to build temporary harbours off the coast of Normandy.
Split into simpler sentences: One of the major challenges facing the D-Day landings was to secure a European port for the delivery of essential supplies. But instead of capturing an existing port, the Allies decided on an audacious plan to build temporary harbors off the coast of Normandy.