Eren's "plan" shouldn't work though.
How it would play out, the largest nations would take out the wall of titans heading towards them and regroup/rearm while the titans take out the impoverished countries that couldn't defend. A few more airstrikes and well placed naval artilleries would keep the remaining titans at bay and land/rail weapons could dispose of them easily.
An army of beast or cart titans would be a lot more dangerous.
None of this would work. Nothing would. You don't get the scale of things here.
There are at least several hundred thousand giant titans inside the walls, quite possibly millions. Anti-Titan technology has only
just recently become a big thing. There are maybe a few hundred heavy-caliber artillery pieces capable of reliably one-shotting standard-size Titans in the entire world, and only a few dozen airships. In real life there never more than a few hundred battleships afloat at any one time, far more than we ever see in the AoT universe, which is still only at early-WW1 era technology. You could assemble all of these resources in one place and they would perhaps kill... fifty to a hundred Wall Titans, then either run out of ammunition or be overrun and crushed.
Minefields wouldn't work either. You would need gigantic mines to even blow the legs off a Wall Titan, which it would promptly regenerate - Titans can regrow
anything so long as their napes are intact, and on a Wall Titan, that is a weak-spot of utterly minuscule proportions no weapon in the world can reliably hit.
Even a thousand Wall Titans could reliably destroy most of the world without support. Ten thousand could accomplish the Rumbling by themselves, just relatively slowly.
If anything the dumbest thing Erin does is bring down
all the Walls, an act with is
explicitly stated to have killed
thousands of entirely innocent Eldians from falling debris. He could easily have destroyed the world with the Titans in Wall Maria alone. Hell, he could've done it with
a quarter of the Titans in Wall Maria.
The Wall Titans are an apocalyptic force. There is
absolutely nothing whatsoever the nations of the world could have done to stop them, period.
