How to win against a numerically superior opponent

Does anyone have any ideas on how to overcome a relentless spam of Infantry, Tanks, and Bombers? Turn-blocking can only get you so far in delaying an enemy advance. I'd like to hear your experience fighting an opponent with 3 times or more troops at their disposal with you having limited resources.

Usually you lose. The only chance you have is to cap their capital and kill them that way. Unless there is a massive skill difference, you can't win against a bigger enemy.

It is very easy, in my experience.

I play world maps all the time and occasionally someone picks China, then he acquires all of Asia and spams thousands of infantry using the IMP strategy. There are several solutions depending on your situation.

First of all, you need to build up huge quantities of defense and make it look as though you're trying. But that's the decoy. Normally people who play with such mindless strategies are autistic and have autistic strength. This materializes in the real world when autistic children are often able to overpower other children despite their lacking intelligence. Use their weakness to your advantage. You have to wall everywhere. Do not RP wall unless the other player clearly lacks commitment. You have to wall like crazy and wall his units in with multiple layers. This will tire him out, then you can go in for the kill turn 50. If he has more SP than you, then you're fucked

If you don't want the hassle of making him click an IP grabber and then ddossing him, try walling, defending (unless you are blitz) and sending stealth to his cap.

You can send an insane amount of marines to their cap and hope they won't have time to reinforce it.

Can think of the French invasion of Russia in 1812 as analogous. Napoleon's Grande Armee is the At-War mega stack and you're the humble Russian trying to hold it all together. Like their Cossacks operated behind the French lines to deny supplies/reinforcements so can you with marines.

I like to use marines to roll up an advancing mega stack's gains from behind whilst denying them any reinforcements in countries they take.

E.g. the stack will attack country x, you buy all the reinforcements in that country (go for the cheapest, militia). Place just enough marines behind the stack that you can continually re-take countries after it moves on. Only do retake in the reinforcements round though. Like this you isolate the stack, deny it any reinforcements and it should melt away like Napoleon's army in Russia did according to this graph, with the size of his army represented by the thickness of the column:
https://upload.wikimedia.org/wikipedia/commons/2/29/Minard.png