I seek counsel

[MarshalN]

Don't make alliances!! Unless you're desperate and have little chance of winning, don't do it. It limits your options and if you make a seperate peace before the alliance runs out, the civ with whom you had the alliance will hate you.

[Dry]

As long as you do not attack one of his city or one of his units on his territory, you have nothing to fear from war weariness, so don't switch, stay in Republic.
If you expect the war to be long, try to find an ally. The best is the one on the opposite side of your ennemy, he will have to fight on 2 fronts. Write down the year of the alliance, because you will not be able to make peace with your ennemy within the 20 turns without consequences on your alliance.
Usually, your ennemy will find allies, find again yourself allies against them so you don't have to care about those. One for one is usually enough. But don't wait for that, or your ennemy will be able to ally the whole world against you. Focus on 1 ennemy at the time.

The odds are against you, so try to find something to your advantage: terrain.
Let him come on your territory. On your territory, you will have the benefit from roads, he not, you will be able to heal, he not.
All your moves should be dictated by the 'allow the wounded to retreat' motto. Attack only with mobile units that are able to move (retreat) after combat.
His dangerous units are the mobile units, so go for them first.
This (go first) should not be a big deal as they will usually come first, before his infantry. But, this process is the bloody one for your own mobile units, so think to that when choosing what new units to build.
Once he has no more mobile units, your mobiles should be safe.
Infantry units are slower and take even the time to destroy improvements. Use this time to move your mobile units to the hot points, but also to move defenders according to threats. Small stack are not a big problem, they will not attack, just destroy your roads. But, eventually, a big dangerous infantry stack will come. Try to weak it with your mobiles. If it nears some city, bring more defense. Usually, 3 strong defenders (non-mobile), will dissuade him to attack. He will instead try to go for another city. You may play the game of "3 defenders here, 3 defenders there", and he may change his mind each turn. Continue to harras him with your mobile units until he is dead.

If you are sure a city will fall, evacuate it, save your defenders to strengthen other cities and if it is not on your future counter-attack path, you may consider giving it to some neutral power (*not* to your ally: the city is empty!!!). Either, he will attack this neutral - you win an ally -, or he will not benefit from the roads - you win time.

After a while, the tide of ennemy will weaken, and he may even ask for peace. It is up to you to accept or refuse. I usually refuse, because I know he is weak. His own cities are now poorly defended and I usualy conquer him in less than 12 turns.

War weariness you said? What is it? Never heard that?

BTW: have you a saved game of this. I would be interrested.