Land combat is pretty straightforward in most real-time games, where you try to amass overwhelming numbers and send them at your enemy. The same is true in Supreme Commander, but there are a couple important caveats to remember when thinking about land combat in this game:
1. The maps are very big.
2. Tanks are very slow.
As Chris Taylor puts it, "you can't just build in Moscow and click on Paris." Air units and even certain naval units can traverse the massive Supreme Commander maps quickly, but the biggest guns that you can produce in bulk will have to travel across the land. So the question becomes: how to get them there?
Transporting your heavy units is a big part of Supreme Commander. The huge flying transports we mentioned in our preview can carry over a dozen small infantry robots or a pair of heavy tanks each -- you'll have to build a whole wing of them to quickly move an entire squadron of tanks, but the ability to transmit that much force all over the board will be worth it.
There are plenty of ways to use ground troops to surprise the enemy. Taylor was so excited he was practically drooling as he described invading an enemy beach using submarine troop carriers. They'd breach the water like great whales, then slowly open while taking heavy artillery fire from shore batteries, until at last your tanks and troops would pour out. Don't want to mess with submersible transports? Some factions may be able to build amphibious tanks that would simply claw their way out of the sea.
Next: Static Defenses and artillery...
If you are making the long trek across the land with a huge ground force, you'll need to protect it. A whole array of support vehicles will be available to all factions. Radar jammers will obscure your radar signature, mobile flak cannons can provide protection against aerial assaults, and mobile shield generators can deflect incoming fire.
Static offensive and defensive placements will naturally factor into Supreme Commander, so once your army does arrive at an enemy base, expect to pound your way through his defenses. All factions can build walls and defensive towers. The alien-like Aeon faction has some particularly nasty technology: walls that appear flat will suddenly come alive as hidden guns fold out to crush your assault.
As for artillery, the arsenal in Supreme Commander runs the gamut. Smaller artillery pieces are mobile and can move around the battlefield, settling down into a bracing position in order to fire. Mortars are also available, which will lob shells high in the air -- this is perfect for battling enemies that are close but behind an emplacement or high terrain. As you'd expect, the biggest of the big guns are obscene -- enormous static cannons capable of hurling massive shells across the map. They may not be accurate, but they can keep firing as long as you can keep them supplied. Keeping your base and your forces hidden from these beasts is essential.
As with all the units in the game, the three different factions in Supreme Commander each have their own look and 'feel.' Earth forces look the most conventional, with lots of wheels and tracks. The half-man half-machine Cybran race prefers devices that walk (such as the massive experimental spider shown in many screenshots.) The Aeon's, meanwhile, prefer sleek smooth metallic designs, and many of their land vehicles hover in order to move.
Regardless of your strategy, naval, nuclear, or air warfare probably won't finish your opponents off. You're going to eventually have to bring in the land troops to mop up.