It wasn’t gentlemanly. That’s why the invasion of Belgium caused such outrage.
Other than that, there are very few examples; Portugal, Romania and Bulgaria all joined in voluntarily to grab tt; Persia was already considered a “protectorate” by the UK & Russia; Greece invited the Allies in; Albania was in a state of anarchy and scarcely had a government to overthrow; Austria and Serbia each accused each other of starting the whole thing.
The other neutrals pretty much minded their own business and made a quiet profit from the war.
By and large, not giving the defending forces to the enemy should be enough to discourage attacks in a game which should be finely balanced between existing units.