Here is my proposal. It is not a decision, but rather a place to start discussion.
Part One: The Aztecs
I think we should conquer all but one Aztec city, and then sue for peace for everything they have. We should attempt to leave one Aztec unit in our territory when we take the second to last city. On the next turn, we should tell them to remove their unit or declare war. There is a good chance that they, in their arrogance, will declare war and allow us to finish our conquest and get all of their techs.
Part Two: The Japanese
We must eliminate or seriously weaken the Japanese before the onset of chivalry, or we may find ourselves being driven back and conquered. I suggest that within 3 turns of the completion of our conquest of the Aztecs, we begin moving 3/4 of our military towards the Japanese, and declare war as soon as enough troops to take one city have reached their border.
Part 3: Entangling Alliances
At this point, we should keep enough money in reserve to sign an alliance against the Americans should they decide to either encroach too far into our territory or simply take advantage of the situation and declare war. This alliance should be with their closest strong neighbor, to keep them occupied while we finish with the Japanese. The war with Japan will likely take upwards of 20 turns, so for each ally they drag into the war, I suggest we drag one in as well, but have our ally declare war on their new ally. This will cause general chaos on the continent, and should give us some extra time to secure our area to the West of the landbridge, which we can then defend easily. To accomplish this part of this plan, we need maps so we know who we should pit against who. Thus, I formally request funds from the domestic department to purchase a map from one of our neighbors.
Part 4: The Americans
I can almost guarantee that the Americans will infringe on our territory. Hopefully, we can wait until the Japanese have been annhilated or at least weakened significantly before we have to declare war on them. There is a very good chance they will save us the trouble of making that decision by declaring war on us, anyway. Regardless, we will want to declare war almost immediately after the war with Japan has ended, and troops should be moved into position before this occurs.
Finally, it may not be possible for us to defeat the Japanese in our first war. If this becomes apparent, all of our resources should be devoted to capturing all of the iron on the continent, to deprive them of Samuari for as long as possible.
Also, while we know nothing of most civs, I believe that the Aztecs and Japanese should be treated as enemies as far as trade goes beginning now, and that we should only trade with the Americans if the deal is much better than we can get elsewhere. All other nations should be considered potential trading partners.
Discuss...

Edit: Added part 4. Thanks, SAaM for reminding me.
Part One: The Aztecs
I think we should conquer all but one Aztec city, and then sue for peace for everything they have. We should attempt to leave one Aztec unit in our territory when we take the second to last city. On the next turn, we should tell them to remove their unit or declare war. There is a good chance that they, in their arrogance, will declare war and allow us to finish our conquest and get all of their techs.
Part Two: The Japanese
We must eliminate or seriously weaken the Japanese before the onset of chivalry, or we may find ourselves being driven back and conquered. I suggest that within 3 turns of the completion of our conquest of the Aztecs, we begin moving 3/4 of our military towards the Japanese, and declare war as soon as enough troops to take one city have reached their border.
Part 3: Entangling Alliances
At this point, we should keep enough money in reserve to sign an alliance against the Americans should they decide to either encroach too far into our territory or simply take advantage of the situation and declare war. This alliance should be with their closest strong neighbor, to keep them occupied while we finish with the Japanese. The war with Japan will likely take upwards of 20 turns, so for each ally they drag into the war, I suggest we drag one in as well, but have our ally declare war on their new ally. This will cause general chaos on the continent, and should give us some extra time to secure our area to the West of the landbridge, which we can then defend easily. To accomplish this part of this plan, we need maps so we know who we should pit against who. Thus, I formally request funds from the domestic department to purchase a map from one of our neighbors.
Part 4: The Americans
I can almost guarantee that the Americans will infringe on our territory. Hopefully, we can wait until the Japanese have been annhilated or at least weakened significantly before we have to declare war on them. There is a very good chance they will save us the trouble of making that decision by declaring war on us, anyway. Regardless, we will want to declare war almost immediately after the war with Japan has ended, and troops should be moved into position before this occurs.
Finally, it may not be possible for us to defeat the Japanese in our first war. If this becomes apparent, all of our resources should be devoted to capturing all of the iron on the continent, to deprive them of Samuari for as long as possible.
Also, while we know nothing of most civs, I believe that the Aztecs and Japanese should be treated as enemies as far as trade goes beginning now, and that we should only trade with the Americans if the deal is much better than we can get elsewhere. All other nations should be considered potential trading partners.
Discuss...

Edit: Added part 4. Thanks, SAaM for reminding me.
