Complex sentence: By remaining neutral during World War II, Ireland ensured that Britain did not regain naval rights to the ports that would have provided either Britain or Germany exceptional control of the North Atlantic if they were attacked and captured.
Split into simpler sentences: By remaining neutral during World War II Ireland could ensure that Britain would not regain naval rights to these ports. The location of the ports would have provided either Britain or Germany exceptional control of the North Atlantic if they were attacked and captured.