battle of somme

This is straightforward. The strategy has been laid down in the dialogue: Take the southern cities to develop Military Academy (to make and spam Spy and/or Blast, followed by Air Bombing), while building / upgrading only University.

I dropped off the breastplate cav gen on the northern bank while the rest will land on the southern bank.

The northern bank relies on pinching off the defence line, before hitting the fortresses hard. Wipe out all remaining troops.
Note: Foch's troops will move on Turn 3/4.
Then, pick off the northern fort of the 2nd defence line before moving to the south.
Lastly, take the city.

The southern bank relies on you breaking through the enemy counterattack and taking the city ASAP.
Tear down the stables, especially in Hard. I did so to build a small fort to block the German swarm.
Use Spy + Blast and quickly knock out the fort. Then, I kept the inf gens around to defeat the mobs while my tank gen attacks and takes the SE city withou trouble. Spam Blast + Air Bombing to take it.
Note: Deploy about one light machine gunner to attack the enemy arty and/or breastplate cav. Then, you should be able to hold using militia or light inf.
Lastly, support the northern city using Air Bombing.

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Note: I tried this in Hard using 3rd class gens and beat it by Turn 11, before the mentioned "reinforcements" arrive.
If enemy reinforcements showed up, think about whether you can take the cities quickly.
If unable to, pull back, regroup and fight them off.