Late game modern era wars are the kind of wars you will most likely be fighting when you play on the higher scale difficulty, mostly because the AI advantages are so pronounced in the early game that engaging in any meaningful war, outside of short opportunistic wars is usually not in the cards.
In the modern era, my advice is nothing radical and part of it has already been mentioned. Artillery, lots and lots of them as their accuracy aren't the best in the world. I would also add several things.
- Bombers. These function like artillery but also has a bonus of its range and its ability to kill units (post C3C). You can pretty much cover your entire empire if you concentrate your bombers on a few key cities. The use of airpower in the modern era is often understated, but the advantages are obvious. In addition to range, you can augment your artillery with these suckers. If your arty stack just couldn't redline the AI's SoD, having extra bombers is great. Even better is if your bomber coverage overlap, and you can use bombers from not one, but two or three cities to bomb the attackers.
Another bonus with bombers is if you're trying to save your precious attacking units (tanks, cavalry, modern armor) and don't want to risk attacking that mech inf. on the mountain with a 100% defense bonus. In C3C, bombers can kill off these units, and they can get promoted too, making them harder for the AI fighters to shoot down when you send them on bomber runs into enemy territory.
Another point to learn post Railroads is to keep a stategic reserve.
I usually stack all my surplus units outside cities, so I'd have a stack fo Defenders, Attacks and Bombard units somewhere safe. And I pull units from these stacks where needed. The strategic reserve is key, because the AI can send SoD from multiple fronts and if they have an RoP with one of your neighbours, those can show up anywhere. The last thing you want is to be caught unprepared with all your troops in the front.
Lastly, assess the situation early and compensate early. My last game had me short on rubber, which meant I can't build tanks or infantry. I immediately traded my neighbour for 20 turns of rubber and made as many of those two as I could and that saved my behind and bought me just enough time. In the modern era, Modern Armor is going to eat through your stacks on Mech. Infs in your towns, so look to build walls on your towns and build civil defense (ptw and c3c) in your cities and metropolis. I've lost stacks of 4 mech inf. in cities I thought were safe, and lost a town because of it. If possible, plan early and build some of your cities on hills to get the defense bonus.