It's not all 20 million at once, but after a couple of years, maybe. But even if so many soldiers were gathered and thrown at once, they have neither weapons, nor food, nor clothes. And over time, they need to be repaired, cleaned, transported and heated (which, by the way, requires fuel, and I mean a LOT of FUEL). It takes a lot of doctors (who need medicine to treat them. And that's just to keep the fighters alive, not to mention a resource to move forward. There will be hundreds of thousands, if not millions) of casualties. Just imagine such chaos in 20 million people. It won't be like a children's camp. There will be cold nights, enemy partisans who will shoot and cut them. COVID-19, tuberculosis and who knows what else. This is a notoriously unsuccessful plan of attack. This is pure intimidation.