The old rule against killing rival leaders has broken down, and that is bad news for anyone in power, including American presidents.

• Governments used to avoid targeting each other's leaders because it kept everyone safer at the top.
• Big powers liked that rule because it pushed fights onto the battlefield, where their money and armies decided the outcome.
• Killing leaders is a weapon of the weaker side — cheap, and it skips the war entirely.
• The killings of Gaddafi and Saddam, and the strikes on Iran's senior commanders at the start of the Iran war by the U.S. and Israel, show the rule is gone.

Outlook: Leaders on all sides, U.S. presidents included, will have to treat themselves as targets in a way they did not a generation ago.