Complex sentence: Following the War of 1812, the United States again came to realize the importance of protecting Hampton Roads and the inland waters from attack by sea, and construction was begun in 1819 on what would become the largest stone fort ever built in the United States.
Split into simpler sentences: Following the War of 1812, the United States realized the need to protect Hampton Roads and the inland waters from attack by sea. It began construction in 1819 on what would become the largest stone fort ever built in the United States.