Complex sentence: Shafter's plans to attack Santiago de Cuba called for Lawton's division to move north and reduce the Spanish stronghold at El Caney, which was to take about two hours then join with the rest of the troops for the attack on the San Juan Heights.
Split into simpler sentences: Shafter's plans to attack Santiago de Cuba called for Lawton's division to move north and reduce the Spanish stronghold at El Caney, a task which was to take about two hours. Then they were to join with the rest of the troops for the attack on the San Juan Heights.