Complex sentence: The arms races of this period produced the military equipment which eroded the attacker's advantage during Europe's "Long Peace", giving decisive advantage to the defender and setting the stage for the catastrophic stalemate of the First World War.
Split into simpler sentences: The arms races of this period produced the military equipment which eroded the attacker's advantage during Europe's "Long Peace". It gave decisive advantage to the defender and set the stage for the catastrophic stalemate of the First World War.