The Riverboat Relay

A marching column reaches a river with no bridge. On the bank, two boys have a tiny boat. The boat can safely carry either both boys together or one soldier alone. It cannot carry a soldier with either boy, and nobody can swim. The soldiers need to cross one at a time, while the boys operate the boat so the same procedure can be repeated for every soldier. Starting with the boys, the boat, and the army on the same bank, how many one-way crossings are needed to move one soldier across and leave both boys and the boat back on the original bank, ready to help the next soldier?