Wikipedia is your friend, while not always accurate it is the majority of the time. It also makes a better search engine than Google in my opinion since much of the information has links. Try looking up Civil Defense, Bunkers/Fallout Shelters, Nuclear Warfare, Ballistic Missile Defense and make sure to check out everything related to these subjects. This will give you a decent grounding in nuclear strategy in only a few hours.
If your troops posses the capability to fight in this environment, combined with ballistic missile defense, offensive nuclear and chemical capability, a massive civil defense effort (better to spend your billions on bunkers and fallout shelters than massive forts just begging for a missile strike) a informed and trained populace (maybe Nuclear War Survival Skills is on your 6th grade reading list) then perhaps you could look into using a nuclear defensive strategy. Your opponent seems to lack BMD apart from his navy and a couple of nuclear tipped torpedos can do away with that, this leaves him vulnerable to nuclear checkmate (You shoot down his ICBM/SLBM/Cruise Missiles and Bombers leaving him weaponless and open to your own Nuclear weapons) Tactical Nuclear weapons can vaporize his troops concentrations or at least force him to fight dispersed. Better yet If you strike his Invasion fleet(s) with Tac Nukes while his armys are still on the boats than you kill 2 birds with 1 stone.
To secure your factory's than you could do the following. 1. Move them away from the coast to keep them from the advancing enemy, the Russkies did this in WW2 moving entire factories hundreds of miles behind the Urals to keep them in operation. 2. Disperse your production capability as much as possible, Japan in WW2 moved much of its weapons production capability into the homes of its workers. 3. Protect your factories inside of an air defense sanctuary, Interceptors and SAMs can make it costly to destroy them for the enemy. 4. Put your factories in Hardened underground bunkers and try and keep there locations secret. Also Don't forget to keep your workers safe from enemy attack (see Civil Defense). Vehicles moving supplies to and from your factory's will be vulnerable to air attack and will lead your enemy's right to them. If possible move as many pieces on foot and assemble the product away from the factory.
Walls, Land mines, Pillboxes, Barbed wire will all just slow your enemy down and unless artillery and Anti-Tank weapons are present than it might not slow them by much. In this modern age of precision guided weaponry forts and strongholds are likely to be destroyed without every seeing the enemy. Worse yet most will be ignored as your enemy will not land everywhere at once .Strong Mobile forces are far more useful than fortresses. If you truly wish to have fortresses build them around your ports to make it difficult for the enemy to seize them.
Strike your enemy's supply lines and when every you see him massing for an attack preempt him by attacking. Try and always strike him where he is weakest.
Don't let him have any areas of value without paying for them dearly, empty desert is not worth fighting for. Finally If you can't have it, blow it up or spray persistent chemical agents on it to keep it from him.
If you have anymore questions please be specific in asking about them for example How should I target my strategic missiles or how does modern air defense warfare work. Look around the forum as many of your questions were almost certainly already asked some time ago and a long and tedious debate will have formed on them with plenty of suggestions and alternatives just waiting to be exploited. Learning your acronyms will help you decipher many conversations
Remember there are multiple schools of thought when it comes to most of this stuff, 90% of the time someone will come along and say that the guy who was helping you was wrong or that there is a better way to do it. Just look at a thread on Russian tank design vs Western Tank Design philosophy for an example
Finally always try and Google or wiki your question first (There is no such thing as a stupid question but if the information is already available why ask?)
