"The law of unintended consequences is the only real law of history." So says my good friend, the distinguished historian Niall Ferguson. We are seeing the wisdom of that line playing out in the days since a United States drone strike killed Iran's most important military commander, Major-General Qassem Soleimani.

Certainly Maj-Gen Soleimani was a dangerous, smart and determined enemy of the US. Taking his skills away from the Iranian regime is a tactical plus in the effort against Iranian aggression and Middle East terrorism. Indeed, Iran's ability to respond to his killing is now handicapped - the best person to imagine, plan and oversee a revenge operation would have been Maj-Gen Soleimani himself.