I think the only real ethical difference is when a standing army is used as a fallback when the diplomacy fails. If the attack comes only when there's a genuine need, and the diplomats have been genuinely unable to resolve the problem, then that is ethically different from terrorism. (It may not be ethically defensible, but it is different.)
If, on the other hand, the political leaders pull the diplomats out before they have a chance to finish their work, and say, "See, we have to send in the army now", then that's ethically the same as a terrorist.
Of course, this equality applies only when you're talking about total war. These days the US tries to pretend it's not engaging in total war, but we can't completely avoid killing civilians. Rumsfeld said the other day that the US killed 1500 (2500?) Iraqis in August; he said they were terrorists, but if even 20% of them were misidentified, and if that rate is typical of the war so far, then the US in Iraq has killed more civilians than died on 9/11.
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If, on the other hand, the political leaders pull the diplomats out before they have a chance to finish their work, and say, "See, we have to send in the army now", then that's ethically the same as a terrorist.
Of course, this equality applies only when you're talking about total war. These days the US tries to pretend it's not engaging in total war, but we can't completely avoid killing civilians. Rumsfeld said the other day that the US killed 1500 (2500?) Iraqis in August; he said they were terrorists, but if even 20% of them were misidentified, and if that rate is typical of the war so far, then the US in Iraq has killed more civilians than died on 9/11.