Diplomacy (n.) and the rule of law are invoked most eagerly when the offender is an official enemy.
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Diplomacy (n.) is the enemy of a power that is militarily strong but politically weak, since any settlement moves the contest onto ground where it loses.
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Diplomacy (n.) can be mere theater, staged to cover the time it takes to move the troops into place.
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Diplomacy (n.) can be a way of buying time for a war: a peace envoy who meanders through the Mediterranean while the offensive he was sent to stop finishes its work.
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Diplomacy (n.) comes, for war planners, "after, but only after" a clear pattern of pressure: first violence, then perhaps peace.