A reason why Poroshenko returned to Ukraine may be that he will call for Russian troops to stabilize Ukraine as an unrest in Ukraine has begun, staged by Russian special forces pretending to be Ukrainians.
As Russia hasn’t accepted how Poroshenko was deposed, so they could say he is the President of Ukraine instead of Zelenskyy and those they are responding to the “legal leader” when they cross the border.
This will be in a way a copy of the Nazi German invasion of Poland, just faked facts to make your aggression “valid”. We just have one criminal to thank for the situation with Ukraine and that is Putin, the only proper place for him is hanging from a ligthpole next to the Red Square.