For a period in the late 19th century, naval tactics became chaotic as ironclad warships were introduced. One school of thought held that ironclads were effectively invulnerable to gunfire, so ramming became a popular method of attack, such as at the Battle of Lissa and the Battle of the Yalu River. Another held that naval battles would only be decided by an assault on an enemy fleet in port. Ships built according to these doctrines tended to mount a handful of guns which could fire ahead or all-round, rather than broadside. The fleets of these periods tended to use the line of battle less.