The trade tab lies within the Foreign Advisor/F4 screen. For example, it can look look like this, if you have embassies with all of the civs in your game:
As a general strategy, once you have Navigation and the world map and a harbor, consider signing a military alliance with whomever has an active trade route (the blue lines above) with the nation that you seek to conquer. Or if you feel 20 turn military alliances might be worth it for your defensive war. If your target AI cannot trade for luxuries with other AIs, then they end up reduced to whatever happiness/content-ness structures they have, and their native luxuries only. They also cannot import strategic resources. Training cavalry might no longer be possible for the enemy if you follow the above "no trading with the enemy" plan. Also, instead of just signing a military alliance against the target AI, I advise checking with those you just signed the military alliance to see if you have any new luxuries or resources that you might trade for.
For example, with the above screenshot, suppose we wanted to go to war with Greece. Then, we would sign military alliances with Korea, Persia, and Babylon. If the war will last less than 14 turns, you might also make a profit on the war by first loaning out GPT with your lump sum of gold from say Korea. Then use more GPT to pay Korea for the military alliance and the gold you just lent out. Then, when the enemy leader gets eliminated, Korea will still pay you GPT.
The trading tab has further use when you lack a luxury or strategic resource. In the following save (corresponding to the above screenshot), the Maya lack ivory:
Suppose that we wanted to get that ivory without conquering an AI. Who would we need to trade with? Looking over Greek territory, using Ctrl + Shift + M as needed, we can see that Greece has 3 tiles of ivory hooked by road networks to their capital. But, the trade advisor tells us that Greece has no extra ivory. The trade tab thus tells us that Greece exports ivory to Babylon, Persia, or Korea.
Question 1: Does another AI have their own source of a luxury or resource? For example, does Persia have ivory?
Babylon does not have any ivory source. Persia does not have any source of ivory. Korea does not have any ivory. If any had ivory, then we could eliminate that AI from consideration of having ivory imported from Greece. But, in this case we can't eliminate anyone from consideration this way.
Question 2: Who has ivory?
Let's try a city investigation with Babylon. We'll use their capital city of Babylon, since other cities aren't necessarily hooked to the trade network in general. But, the capital always exists on the trade network if anyone (you or the AI) imports a resource or luxury:
We find that Babylon has ivory:
Investigating Seoul we find that it has ivory also:
So, we predict that Persia does not have ivory. Luckily for us also, Babylon and Korea do not have any other trading partners, and thus could not import ivory from anyone else but Greece.
Now that we know Greece exports ivory to both Korea and Babylon, we would have two options for trading for the ivory from Greece.
1. Declare war on Korea. Then ally Greece against Korea.
2. Declare war on Babylon. Then ally Greece against Babylon.
Under either plan, Greece should then have ivory for sale (and I tested it for Babylon). For the following save, it may not have fit with overall strategic plans to declare war on Korea or Babylon before having Greece declare war on The Maya (or as non-equivalent alternative) declaring war on Greece. For starters, The Maya have an RoP with Babylon, and one with Korea also, and declaring war on someone with an RoP implies a reputation hit. But, still the military alliance with either would enable trading for ivory.
The other luxury lacking for the Maya is Gems. The Byzantines, the Koreans, Persia, and Russia all have at least one source of gems, since it says Gems in the leaderhead screen:
But, all of them only have one source of gems, so trading for gems would be impossible on this turn.
Special note: All strategic resources or luxuries that an AI has will appear in greyed out text, if you contact an AI with no trade route to your capital. You may have a source of iron, for example. But, an AI might have extra iron available to trade if you disconnect your iron source.
Final note: The example with Babylon, Greece, and Korea did involve city investigations. But, city investigations aren't always needed to figure out who supplies what to whom. That can also get guessed at by occasional use of the trade tab.
As a general strategy, once you have Navigation and the world map and a harbor, consider signing a military alliance with whomever has an active trade route (the blue lines above) with the nation that you seek to conquer. Or if you feel 20 turn military alliances might be worth it for your defensive war. If your target AI cannot trade for luxuries with other AIs, then they end up reduced to whatever happiness/content-ness structures they have, and their native luxuries only. They also cannot import strategic resources. Training cavalry might no longer be possible for the enemy if you follow the above "no trading with the enemy" plan. Also, instead of just signing a military alliance against the target AI, I advise checking with those you just signed the military alliance to see if you have any new luxuries or resources that you might trade for.
For example, with the above screenshot, suppose we wanted to go to war with Greece. Then, we would sign military alliances with Korea, Persia, and Babylon. If the war will last less than 14 turns, you might also make a profit on the war by first loaning out GPT with your lump sum of gold from say Korea. Then use more GPT to pay Korea for the military alliance and the gold you just lent out. Then, when the enemy leader gets eliminated, Korea will still pay you GPT.
The trading tab has further use when you lack a luxury or strategic resource. In the following save (corresponding to the above screenshot), the Maya lack ivory:
Suppose that we wanted to get that ivory without conquering an AI. Who would we need to trade with? Looking over Greek territory, using Ctrl + Shift + M as needed, we can see that Greece has 3 tiles of ivory hooked by road networks to their capital. But, the trade advisor tells us that Greece has no extra ivory. The trade tab thus tells us that Greece exports ivory to Babylon, Persia, or Korea.
Question 1: Does another AI have their own source of a luxury or resource? For example, does Persia have ivory?
Babylon does not have any ivory source. Persia does not have any source of ivory. Korea does not have any ivory. If any had ivory, then we could eliminate that AI from consideration of having ivory imported from Greece. But, in this case we can't eliminate anyone from consideration this way.
Question 2: Who has ivory?
Let's try a city investigation with Babylon. We'll use their capital city of Babylon, since other cities aren't necessarily hooked to the trade network in general. But, the capital always exists on the trade network if anyone (you or the AI) imports a resource or luxury:
We find that Babylon has ivory:
Investigating Seoul we find that it has ivory also:
So, we predict that Persia does not have ivory. Luckily for us also, Babylon and Korea do not have any other trading partners, and thus could not import ivory from anyone else but Greece.
Now that we know Greece exports ivory to both Korea and Babylon, we would have two options for trading for the ivory from Greece.
1. Declare war on Korea. Then ally Greece against Korea.
2. Declare war on Babylon. Then ally Greece against Babylon.
Under either plan, Greece should then have ivory for sale (and I tested it for Babylon). For the following save, it may not have fit with overall strategic plans to declare war on Korea or Babylon before having Greece declare war on The Maya (or as non-equivalent alternative) declaring war on Greece. For starters, The Maya have an RoP with Babylon, and one with Korea also, and declaring war on someone with an RoP implies a reputation hit. But, still the military alliance with either would enable trading for ivory.
The other luxury lacking for the Maya is Gems. The Byzantines, the Koreans, Persia, and Russia all have at least one source of gems, since it says Gems in the leaderhead screen:
But, all of them only have one source of gems, so trading for gems would be impossible on this turn.
Special note: All strategic resources or luxuries that an AI has will appear in greyed out text, if you contact an AI with no trade route to your capital. You may have a source of iron, for example. But, an AI might have extra iron available to trade if you disconnect your iron source.
Final note: The example with Babylon, Greece, and Korea did involve city investigations. But, city investigations aren't always needed to figure out who supplies what to whom. That can also get guessed at by occasional use of the trade tab.
