Tanks do more damage to everything on the ground BUT are best used in opening terrain.
Every unit has a close combat armor value and close combat attack value which is different from the normal attack and armor value. You'll notice a tank's close combat attack value is less than its normal attack and so is its close combat armor value. Infantry on the other hand get a bonus in their close combat attack values. DO NOT send your expensive tanks to take a city or have them engage infantry in mountainous terrain.

Anti air units and air units do more damage to air units than anything else. Your anti air unit can also target other ground units but it won't do a lot of damage.

There are two types of attacks an anti air unit can do against air units; attacking Low Flying air units, like helicopters and fighter jets coming in for a strike, and High Flying air units like strategic bombers and air units that don't come in for a strike but launch missiles.

Some anti air units are capable of dealing damage at both altitudes while others are not. The Technical Readout of a unit is your friend.

Artillery units are your friends and can help to beat back enemy invasions. They also are great for taking cities. Use artillery for defense and invasions by having your infantry/tanks engage the enemy while you're artillery pounds the same enemy units. Artillery units are like candy to air units. The AI will, IF it has them, position anti air along side its artillery units.

There are supply lines. If you have an APC, or a fast scout unit, circling behind an advancing tank column invading your territory it will cut their supply lines. If they don't reconnect the supply lines the invaders will run out of ammo and fuel and then it is a turkey shoot. This way a much smaller army can soundly defeat a larger invading army.

If you're playing as Mexico and the USA decides to invade don't freak out. Yes, the AI of the USA will have a lot of infantry vehicles and tanks heading your way which you can't hope to match in numbers. It will be like the Empire of the Ants coming to devour you. Yes its units will be higher tech than yours. Yes if you engage them you will die. However, if you cut their supply lines and then engage them they will die.

Fight dirty. Have some units engage the enemy and run away trying to draw a group of enemy units after you. Have some units in waiting ready to cut off the pursuers. This requires planning.

If you micro manage your battles like me you have noticed the AI is always trying to flank you and cut off your supply lines. If you're not paying attention a weaker AI force could totally kill your military.

You can bring supply trucks with your invading units which certainly do help. But the AI seems to prioritizes them as targets.

The AI uses supply vehicles also, especially in defense. The AI will position supply vehicles right next to cities you're attacking. Attacking cities from multiple angles will destroy their supply vehicles which are rapidly feeding ammo to its defenders.

No matter how powerful your units are once they are cut off from supply a cave man will always win against your units, always.

Garrisons are extremely weak in this game compared to SRU. Militia units can take out a city garrison now. Garrisoning your cities eats up your reserve personal more than in SRU.

Flanking attacks, attacking a unit or group of units from different sides at the same time, will kill the enemy much faster. It is very important when attacking military bases and cities, with lots of enemy units in them, to attack from multiple sides if you want to take them quickly.

Weather is a factor in this game. From the manual of SRU:
"With the weather system active, areas of the world may become muddy or snow
covered as weather systems cross the map. Ground will freeze, thaw and dry
according to the weather as rain or snow obscure the sunny skies . Weather will
have a variety of effects on units including spotting penalties, reduced movement
speeds, increased reload times, and morale effects from exposure. Experienced
leaders will pay close attention to the weather when making battle plans."