The objective of Bellum se ipsum alet is to influence cities. However, not only do you have to do this in competition with other players, you will also see your influenced cities decreasing in value and even slipping out of your influence. The key to victory is thus to include the time dimension in your strategy. Whenever you are about to capture a city, you should plan ahead for the next city. Ideally, you should have your two leaders capture two cities in about the same turn. This gives you six turns to capture two more cities, after which one leader may return to reoccupy the earliest cities and the other to defend the later cities. That should help you maintaining a stable base of four cities. Let us now see how to handle the different phases of the game.
Stake your territory early. Don't just go for the closest cities but try to establish a "natural" border to your closest enemies. There is no such thing as neutral cities in this game, either you take a city or someone else does. Most likely, the other powers will prefer taking empty cities to fight other powers for cities. You should also aim at a "compact" sphere of influence, where you quickly can move your leaders from one part to another, rather than an outstretched, where you can't defend all parts.
For Sweden, Copenhagen and Dresden are two good starting cities, since this would leave Berlin, Danzig and Breslau behind the lines for later conquests - enough for victory. Austria should claim Breslau early for this reason and also Munich as a border against the French. This would leave the nearby city of Prague for the second wave but since no other power is near enough, it's no hurry to capture this Bohemian stronghold.
France and Spain both have interests in Cologne but should try to avoid early battles. One bold option would be to go for Konstanz/Heidelberg and Utrecht/Brunswick instead to fend off Sweden and Austria before settling the issue.
Clashing ambitions for the opening: Sweden (yellow), Austria (black), France (blue) and Spain (red)
With two cities captured, it's time to look for the third and fourth. One leader can be sent back to occupy/reoccupy cities behind the lines while the second keeps an eye at the other powers and look for an opportunity to snatch a city. You don't want to be the first power to wage a battle but rather the one that grabs the spoils afterwards. If an enemy is approaching, your decision should be determined by the tactical situation. Will the enemy stretch out too far by capturing a city? Then you may want to save your army and take it back later after the enemy's inevitable retreat. Are two enemies approaching? Let them fight and look for gains behind their lines instead. But if the lost city would divide your sphere of influence, you should defend it with full force. You will be weakened but hopefully have time to recover before the other powers take advantage of it. If necessary, bring back your other leader as reinforcement and play the waiting game for a couple of turns. You must be well prepared before the end game assault.
Typically, Sweden should pick up Berlin and (if left alone) Danzig while Austria should go for Prague and look for opportunities to capture Dresden or Heidelberg. For France and Spain, an agreement about Cologne must be made (with arms if necessary), after which Munich and Copenhagen are possible targets.
The end game
By now, you should have two cities recently occupied/reoccupied and another one or two cities with reasonable time left before revolt. You will then have a couple of turns for a quick and sudden expansion to grab the remaining cities required for victory. The other players will most likely do the same so identify the weaker of them and go in that direction. Saving the army is not important now - as long as you get those cities, it doesn't matter if your army is annihilated.
If your expansion fail, your only option is to fall back and let the other players restore the balance. Use all means to point out who is closest to victory and turn them against each other until your army has recovered. By now, some cities are likely to have been ruined, making a city victory more difficult. Instead, there may be opportunities for tactical tricks among the ruins. With hopefully all powers' armies weakened and no city support in the centre, you may be able to cut an enemy leader's retreat and capture him. Remember that leaders also count in the city victory. Another option is to purposely ruin border cities to make advancements from that direction less interesting. If you can maintain your core area established in the mid-game, you still stand a good chance to have the most cities when the game ends.
I haven't mentioned the influence victory so far, where a player run out of influence tiles. The reason for this is that it's more difficult to defend influenced hexes than cities. (A leader needs merely to be adjacent to influenced hexes to have the influence tiles removed.)
A final word of advice: A battle takes place in the first phase of a turn. That means that if a leader moves to capture the eigth city, other leaders must position themselves for battle immediately - if they wait and see if the city falls, they will not have time to take back the city and prevent the victory. This is a very important time dimension to bear in mind, whether you are the conquering or the preventing power. The winner of Bellum se ipsum alet won't be the strongest player but the player who knows when to fall back and when to advance.