“For a bioweapon to work, you have to have a vaccine that guarantees that you dont kill your own army and your own citizens in the process.”
Unless you don’t care if you’re own citizens die from it.
Some people worship death.
You do, however, care about your army, the training you’ve invested, the equipment, the ability to take and hold land, to project power, the ability to garrison, and the ability to prevent attacks.
Even if you don’t give a toss about the populace, you do care about your investment.
q.v. Stalin, Mao, Pol Pot, Hitler, etc.