On June 24, 1314 the Scots defeated the English at the Battle of Bannockburn. Twelve years before the Flemish defeated the French at the battle of the Golden Spurs. The two battles had a telltale tactic and a strategy in common. Both invading armies were drawn to a specific battlefield that neutralized their superior cavalry, and the defenders used a new weapon, a spike that enabled infantry to even the odds against horsemen. On both occasions the invaders were handed defeats that...