I'm sure its happened to the majority of you - you've encountered a player who goes for defense from the offset. In the outstanding majority of games this results in them losing, but a determined and skilled defensive player is a different thing altogether. The most common form of being defensive is known as "turtling" - basically putting a shell around your base (hence the term). This is particularly seen when French players build grand cannons all around their base, and then back them up with various other defensive structures.
There is no such thing as an undefeatable base. It may take time to destroy a large and well defended base, but ultimately, it can be done. The key to winning against such opponents is to not lose your patience. Remember, if you have control of the majority of the map, then it is you who has the advantage. Lots of players hate defensive players. Personally, I don't mind them - if they want to try building a huge base then I say let them. People can do what they want as far as I'm concerned - providing it doesn't involve cheating; so don't 'flame' defensive players, just wipe out that base instead!
The basic defense freak will surround his base, or any entrance to it, with a combination of tesla coils/prism towers, sentry guns/pillboxes, flak cannons/patriots - and, if France, the Grand Cannon. If it was possible to build the undefeatable base, which it isn't, France would be the nation to use. I don't know how many of you have tried attacking a player with 3 or 4 grand cannons, it isn't easy. Obviously I suggest rushing them before they get to this stage, but this is a different section, designed to teach you how to break through a heavily guarded base.
If you have superweapons on, then your task is comparatively easy. Simply send in a nuke or a weather storm on the opponents source of power and then attack him quickly. Remember, defense freaks use a lot of power - if you knock out a few power plants then you will most likely knock all of his defenses offline. Also remember that to build a big base full of high-powered defensive structures costs a lot of credits - if you spend the equivalent on units you should have an impressive arsenal at your disposal. Now, time to tackle those cannons - there are a few tactics that can be used:
The key to destroying Grand Cannons is to distract them and get your units as close to them as possible (this way the grand cannons minimum blast range will be too far).
Ways to distract grand cannons include sending in cheap units - e.g. dogs,gis,conscripts and then letting the cannons shoot at them while you roll your tanks in. Remember, if you put a tank right at the base of the cannon, it will not be able to hit it, and other cannons may still fire at the unit. If they do they will only damage the grand cannon as well.
Another tip is to build a harvester, or two, solely for the purpose of driving it in front of the enemy cannons. Harvesters do take a while to be destroyed by grand cannons, and if you spend 1400 on one, its probably more useful then spending 2000 on conscripts which will die instantly if the cannon fires anywhere near them.
Once you have the cannon distracted, move your tanks in quickly. Fire at the cannon and then carry on doing the same to each cannon if necessary. The key is to break the opponent down gradually, bit by bit, piece by piece.
If you play against an opponent who has blockaded up the only entrance to their base (e.g. maps such as Mount Olympus) then things get a little complicated. I once played against a player who literally built tesla coils all the way across his entrance until it was blocked off (thats about 12 roughly). He then placed about 20 Yuris behind them. A nasty concoction yes, but I was Iraq and I loaded 5 desolators into a flak trak, drove it in front of the coils and then deployed all the desolators as soon as they were forced out of the vehicle. He lost a lot of infantry units. That's a good tactic that can be employed in such a situation.
The basic key to breaking anything down is strength. If you have lots of tanks then you will always victor. But against turtles and defense freaks you must alter the basic strategy of "throw everything you've got at em" - to "throw a little at them and then the rest".
Another extremely important method, that is commonly used, is to "bleed your enemy dry" of income. If they have no harvesters or ore fields to harvest from then you will be able to dwarf their economy. I discussed killing harvesters in the economy section. A real problem can be when a French player builds a base with an ore field in it. But ultimately, you have control of the map if they go defensive - so be aggressive and take out any units he has not in his base. Then finish him off with a big attack (do not attempt lots of small attacks).
Summary: Don't ever lose your patience against a defense freak. Do so and you're doomed. Don't give up if you see a large amount of defense structures. If you control most of the map then you control the game - they are sitting in the base, waiting for you. You are the one who has the option to make the game go whatever way you want it to.