Catherine counterattacks swiftly and with large numbers, but not in the arena I expected. I had grouped my forces on the border expecting her to counterattack around her threatened cities. She does indeed attack in the narrow land region in which I focused my battle plan, but this was anticipated and easily dealt with. However, she also sends two frigates straight to Calixtahuaca, threatening the resources and commerce of my northern cities.
These frigates aren't so ominous themselves, but I also see a large naval invasion force off of Gao, and it is poised to make the same trip.
I failed to consider how quickly this diagonal path across the gulf could bypass the front lines. I was even moving an ironclad away from Calix towards Gao. My rear cities are defended with just one rifle each.

it looks like I can't take on the power leader without getting my hands a little dirty.
I move the ironclad at Calix to cover the fish. I simply don't have enough ironclads to counterattack yet, and there's no way I can use them to prevent the invasion. I move the ironclad at Gao north to cover the coast between the cities. I whip the ironclads in progress at Gao and Timbuktu, and Kumbi will complete another in 2 turns.
To deal with the invasion force itself, I draft in Calix, Djenne, and Athens. Timbuktu and Kumbi already drafted the turn of declaration, and the rifles are still close enough to turn around and protect the coast. I place a rifle on both of the hills south of Calix to make a landing there more difficult. Djenne goes into starvation to build a cannon next turn, and my remaining catapults move two tiles north of Athens and upgrade.
Now I concentrate on the battles I had planned in advance. I have my own invasion force in the jungle outside the gates of Smolensk. Smolensk is defended by 3 rifles and a SAM infantry. I brought 4 grenadiers, 5 cannons, and a medic. My ironclad has already started bombardment and my 2 accuracy cannons finish the job. My 3 suicide cannons are lost, but their death will be avenged!
As planned, we raze the city, and move on to the central arena of the war:
Cathy ignores the undefended forest on the route to Athens, but she also ignores the Woodsmen we were hoping she'd attack. She sends an artillery at our pillaging party, killing the cavalry. We clean up her wounded attacker, and move the remaining pillagers to the farm 1 tile east.
As expected, she also moves towards our territory via the road south of the mountains. She sends two Cossacks, both avoiding the defender on the rice. I dispatch the Cossacks with my rice defender and an Athens rifle. I move a grenadier, rifle, and cav out of Gao to cover the wounded rifle south of the rice. I back them up with two sentries, forming our planned second pillaging party.
Moving forces out of Gao already is maybe prematurely aggressive, but I'm in a reckless mood. I don't move my main army in Athens north to defend....I move it south to attack. Six cannons, five grenadiers, and a medic join my woodsmen to put pressure on Vlad. But I do move my new draft rifle and Athens' defender north, and my last pike goes into Athens to upgrade.
We are making some money now even at 100% science, but it is still not enough to cover the cost of our final upgrades. I talk to our new best friend Saladin:
Giving him Economics for this fee is not a bad deal for us at all. We will want Saladin's trade routes once (if) we end this war and make the post-Astronomy transition. He is in Mercantilism right now, and we need him to switch to Free Trade. I had even been considering gifting him the tech.
But we obviously have some immediate issues to deal with. I'm somewhat prepared for the invasion now. Cathy can't land at the hills south of Calix without some losses, and if she lands further south or goes around to Timbuktu, I'll have time to prepare further.
But Cathy surprises me again:
Since I formed my second pillaging group, I now have only a rifle and grenadier defending Gao.

I had convinced myself that the invasion was following the frigates north to my then undefended interior. And maybe she would've done this if I hadn't blocked off the coast south of Calix, and weakened the defenses of Gao.
The premature formation of the pillaging group was a potentially big mistake. When I was preparing my troops, I was very concerned about an attack on Gao from the desert in the west. This is why I didn't set out my second group of pillagers immediately, I was going to wait till I had enough troops to block this mountain pass.
Cathy's Gao landing may have been a surprise (though it shouldnt have been), but it was not a smart location. I had placed my troops to counterattack an invasion in my interior, and some of them can still attack the near side of Gao. If she had landed on the desert pass, away from my other cities, I would be in very big trouble. Ironically, a desert invasion of Gao strongly influenced my initial placement of troops...I seemed to have forgotten this on the second turn of the war.
I attack the invasion stack with my two cannons upgraded the previous turn, losing both. Then I attack with 2 riflemen and a grenadier, with about 50% odds each time. I get a little lucky and win each battle, killing 3 Cossacks. I attack the stack again with one more gren, losing at 40% odds.
I move a wounded rifle from the previous turn into Gao, and draft another. The mobile part of my pillaging group returns to Gao. A chariot's no counter to SAM infantry, but my city can't be taken if I have more defenders than attackers.
I draft another rifle to protect Athens, should Cathy decide to attack in that direction. I have only two rifles in Athens now, but only her two wounded Cossacks can attack it next turn. Athens is safe with high probability.
I'm satisfied with the invasion situation for now, let's look at the other fronts of the war:
Cathy killed my defending ironclad on the fish, but she lost one frigate, and the other is critically wounded. Like the brilliant tactician she is, she moves her frigate within range of Timbuktu and its newly whipped ironclad. All our sea resources are intact for now, and Timbuktu can continue to research. But Cathy has many more frigates, and the north coast will remain a battleground.
Meanwhile, I see another small invasion force approaching the ruins of Smolensk.
I position my ironclads so she can't land in my territory without losing her ships. If Cathy didn't get the circumnavigation bonus, I could completely prevent a landing and sink her invasion at sea. She'll end up landing the troops on the pigs north of Mauryan, and I'll take out these invaders over the next couple turns with some losses.
It turns out my frantic handling of the Gao invasion in the west was quite successful. I don't lose a single defender, and the massive invasion has been reduced to a single wounded Cossack, which I pick off with a horse archer for laughs.
My own invasion is working a little better than Cathy's.
Cathy has whipped an artillery in Vlad since I began the war, but she inexplicably focuses on the pillaging group instead of the main forest invasion. She only sent one artillery towards the invaders, which was defeated by my Woodsman II grenadier. The battle of Vladivostok is over quickly.
I finish off Vlad with my skirmisher medic, who probably remembers the battle of Sparta one tile away. We keep Vlad, though in a couple turns it will be squeezed between the borders of Thermopylae and Cathy's core cities. So once we have peace with Cathy, we must move our troops to Alex's final city.