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Is war ever justifiable?

If Iran were on the verge of acquiring a nuclear bomb, would that justify a US-led war?

If the Pakistani government lost control of the country, would Washington be justified in sending in troops?

When to employ force is the biggest decision any government makes.

Richard Haass served in both Bush administrations during both US-Iraq wars, Desert Storm in 1991, and Operation Iraqi Freedom in 2003.

He has published a new book where he argues that the former was a "war of necessity" and the latter "a war of choice" that was poorly implemented.

Both wars, he says, offer many foreign policy lessons for US officials.

Haass left the US state department, where he was principal adviser to Colin Powell, then secretary of state, in 2003. Since then, he has been president of the Council on Foreign Relations, one of the US's most influential think tanks.