Taking out Iranian missile launchers striking British targets would be legal. Embarking upon a campaign to dismantle a sovereign state’s long-term military capacity would not. Mr Trump’s objectives are “destroying Iran’s missile capabilities” and “annihilating their navy”, as well as preventing Tehran from ever having nuclear weapons. The US president doesn’t rule out the possibility of American troops on the ground in Iran. If missiles are being launched, shooting them down – or hitting the launcher to stop further attacks – can be lawful defensive action. But this is clearly not what Washington is talking about.