Well, I'm still interested in the ideas of anarres and homeyg. But I think we agree upon the tactic to invade Japan, so I played my ten turns. We are ready for an invasion:
IHT: No civ wants to make peace. And the only civ we are at peace with doesn't want to declare

The Mongols and Americans are in the IA. Persia and Arabia are backward. I will prepare a war with Japan and land as many troops as possible on their continent.
IT: American firgates destroy the barracks and temple of Oslo. The Mongols drop a Keshik on our lands. Japan kicks us out. Alesund: temple->courthouse. America finishes Newton.
Turn 1 (600 AD) trebs redline the Keshik and an elite sword kills it. No leader. The first three berzerks are dropped on the Japanese continent. An archer is upgraded to berzerk.
IT: Trondheim: berzerk->berzerk. Oslo: barracks->berzerk. Stockholm: market->berzerk.
Turn 2 (610 AD) trebs redline a few Mongol and American frigates. Three galleys make it to the capital.
IT: A lot of bombing, but it only redlines units or misses the target.
Turn 3 (620 AD) Two archers upgraded. Two American and a Mongol frigate redlined. At least the redlined ships retreat.
IT: Japan and America trade embargo us

America bombards, but doesn't do much damage. They also drop a musket and MDI. 4 cities: berzerk->berzerk.
Turn 4 (630 AD) Trebs and two elite swords easily get rid of the invasiopn force. But no leaders. Two new archers upgraded. One berzerk send to Japan.
IT: American frigates again miss for the most part.
Turn 5 (640 AD) Trebs again bobmard American Frigates and now 4 berzerks land near Japan. America accepts straight peace. They even want to pay 40 gold
![Party [party] [party]](/images/smilies/partytime.gif)
I do this immediately. Mongolia also is willing to accept straight peace [dance] The two biggest tribes really love us

Compared to the Mongols our army is average in size!! I upgrade another archer.
IT: What a peaceful IT! Except for the impi/MDI pair of the Zulu that lands next to Trondheim, that is.
Turn 6 (650 AD) Two elite swords red-line, but kill the invaders. No leaders. One archer is upgraded and 3 berzerks land near Japan.
Turn 7 (660 AD) Two new upgrades and three berzerks near Japan.
IT: America and Persia embargo us.
Turn 8 (670 AD) 4 trebs and 4 berzerks are dropped on the Japanese island.
IT: The Zulu now invade us with an AC. We finish 3 berzerks.
Turn 9 (680 AD) Trebs redline and an elite sword kills the AC. No leader. The Zulu accept straight peace. I am not afraid of them, so no way. Arabia also is OK with straight peace. But I'm sure they give us war hapiness. So no deal.
Three more berzerks are ready for a battle on the other continent.
Turn 10 (690 AD) 3 berzerks land near Japan. We can now cancel the peace deal and invade them. We have a pike army, 4 trebs and 26 zerks there. We could make some trades, but I decided against it for now. We have to watch out for the income drop when our GA ends.
I don't think we should make peace with the Zulu yet. They give us war hapiness when I'm not mistaken.