As you are aware, there is attrition damage in the game, which can only be nullified using Supply Wagons. However, the way attrition damages you depends on the situation.
The Russian have such high attrition damage that you will not be able to have the forces to take a small city without supply: the attrition will wear away your units before you can penetrate deep into their territory.
Another thing to watch out for is neutral territory. If your soldiers march through neutral territory they will suffer attrition damage about as bad as the Russian Winter, no matter if you have Supply Wagons.
This causes a problem when you must transport soldiers through neutral territory. See "Logistics and Alliances".
When planning an offensive, you must take into account the amount of time necessary to ship soldiers to the battlefield. It is also important to be able to drum up a large (non-Partisan) army in a short amount of time.
Ramping costs will prevent you from building a significant military base in every city. However, there is an effective alternative to sending soldiers from town to town.
You can keep one military base at all times. Whenever soldiers are needed on a different front, raze the military base and rebuild it in a good spot.
The ideal military base, for most situations, is:
5 Barracks
2 Auto Depots
3 Factories
2 Redoubts
3 AA Guns
I call this a Jacobian base (after my first name, Jacob).
Now, there are two things to note about static defenses: three Bunkers have the strength of a single Redoubt, so you may wish to maintain a combination of Bunkers and Redoubts to prevent ramping costs from escalating too high.
Also, make sure to build Lookouts (five) around your Jacobian base and three around every City. This will help defend you from an enemy who has airpower before you even advance to the Industrial Age.
The Redoubts and AA Guns will protect your base from any normal enemy counterattack, allowing you to keep soldiers elsewhere.
Strategic Bombers, and even Bombers, can fly to most corners of the map. However, Biplanes have not enough air to fly deep into enemy territory, and the farther away you send planes the less time they can spend in the air. The solution is to build multiple Airbases throughout your empire. Coupled with using Jacobian bases, this can make your nation much stronger defensively.
You need not maintain a massive air force. Instead, you can build just about 20 aircraft (preferably Bombers) and, when needed, order them to land in the nearest Airbase. You can use this to leapfrog your air force all around your nation.
When defending borders mean everything (next to fortifications).
The shape of your empire in comparison to the competitors will determine your defensive strategy. If you empire only has a small border with an enemy nation, fortify there. If you share a long border, military bases and possibly a garrison are necessary.
Try to avoid having a long, stretched-out nation. This will create problems defensively and logistically, as you can't send troops to one "end" of your nation easily.
Personally, I prefer these defenses for every city:
1 Redoubt (two for a border city)
3 AA Guns
The Redoubt can handle any small force of attackers, while the AA Guns will stop Bombers before they do much damage. Also construct a Temple, and try to build your City up to a Large or Major size.
To succeed, you must be a Machiavellian leader. Make and break alliances in ways that will profit. Forming an emotional fondness towards another nation is okay, but do not be foolish and keep them as an ally if a better ally is available.
There are two scenarios that might arise requiring you to make deals with other nations.
1: Small Passages
If you own a small pass through two neutral nations, or just one neutral nation whose Territory you have cleaved, any soldiers you send through will probably end up suffering damage.
2: Nations Behind Them
Sometimes the nation you want to attack is surrounded by neutral nations. In this situation you will HAVE to do one of these choices:
1 - Give up.
2 - Conquer a nation blocking your pass.
3 - Suffer attrition.
3.5 - As part of Point 3, if you go deep into neutral territory the nation will often declare war on you.
4 - (is possible) Launch an amphibious assault.
Thus, you may in some situations find it useful to break an alliance and forge a new one with another nation. This will allow you to ferry troops through with no problem.
Be warned, though: you have endangered an alliance with another desirable nation.
In some situations, you should "waste" resources on constructing buildings you do not need in cities. There are two principles to this: Increasing City Size and Urban Combat. Both improve your defense. However, these are only to be used if you have plenty of resources.
WHAT EXACTLY IS WASTEFUL?
1. Construction of a Smelter, Lumber Mill, or Granary in a City where you possess no Metal Mines, Woodcutting Camps, or Farms (respectively).
2. Construction of resource camps when you are at the Commerce Cap.
INCREASING CITY SIZE
This couldn't be more clear. You build just enough buildings to increase a City's size, increasing it's hitpoints as well. Do this to border cities to make your border more defendable.
URBAN COMBAT
Just like in the real word, urban combat is frustrating. Soldiers are unable to easily reach the actual City, or fight the defenders. You can turn this to your advantage, by building your Cities into complex mazes.
THESE ARE
These are simply tactics to buy you time to get a defensive garrison there. DO NOT use these as your defense, as having a Smelter or three non-productive Farms will not stop an army from marching in.
[This message has been edited by Random Username (edited 01-20-2010 @ 08:17 PM).]