Your arguing on the implementation of theory into reality though aka the basis of the legality preemptive warfare.
Slippery slope and chalk full of interpretation and debate.
And given the U.N considers it illegal unless of three separate criteria are met, i wouldn't stand behind that military president.
Oh, not even that: even if someone else initiated the war, those tactics and strategies can be used and it remain a defensive-minded struggle. The point is that a defensive war doesn't mean you stay in your own terrain: a defensive war can involve significant expeditionary attempts to weaken the aggressor, and there's no real turn-around to being the aggressor even if you turn it around and occupy their asses. (The most famous being, well, a certain conflict in the early-mid 20th century.)
For a less-than-successful case of a counter-invasion being used as a defensive strategy, the early years of the American Civil War and the eastern theater can apply. Those campaigns, while offensive in nature, were defensive in strategy- significantly more about keeping the north out and off balance than trying to take over it.