Complex sentence: Although naval warfare has a long history, naval mines were not deployed until 1855 in the Crimean War where the first minesweepers were made and consisted of British rowboats trailing grapnels to snag the mines.
Split into simpler sentences: Although naval warfare has a long history, naval mines were not deployed until 1855 in the Crimean War. The first minesweepers date to that war and consisted of British rowboats trailing grapnels to snag the mines.