I like the Swiss deterrent - a well-trained and well-armed yeomen populace with cache of arms and munitions stored throughout the country such that any invader would be faced with a 5th-column of fighters whose sole objective would be to cause maximum damage in men and material of the invader.
I also like the North Korean type nuclear deterrent. It's more expensive, requires a intelligenza and much research and development. But they are a power with the ability to deliver a nuclear attack on anybody foolish enough to invade them. Not so moral a solution, but works well as a deterrent.
I can’t argue about the efficacy. Probably important to have them without a kill switch, so don’t buy the American technology.
That might indeed discourage an attack, make it very expensive.