Before the Conquest, wars between the realms of Westeros were common. Even at kingdoms said to be at peace, neighboring lords would often settle disputes violently. Following Aegon's Conquest, during which he unified six of the seven kingdoms under Targaryen rule, he devised the King's Peace, ending this practice.
Under the King's Peace, liege lords are to adjudicate disputes of their vassal petty lords or landed knight, while those between the great houses of the realm are to be settled by the Iron Throne. Those nobles who go to war without the leave of the King are to be considered rebels and traitors to the Crown. The City Watch of King's Landing is tasked with keeping the king's peace within the city walls.
A Game of Thrones
With the increase of crime in King's Landing due to the influx of travelers for the upcoming Tourney of the Hand, Lord Renly Baratheon, master of laws, threatens to remove Janos Slynt from his post as Commander of the City Watch for his failure to uphold the king's peace.