Knowledge of landscape: +10 for in one's homeland, +5 for warring in an adjacent nation, -1 for low knowledge of an non-adjacent lands which are on you continent, -10 for no knowledge of a distant realm that is not even on your continent and in polar regions (unless the attacker is also polar to) -11.
Armed expatiation in large nations and regions with extremism climates like OTL 2012 Greenland, Niger, Nepal and Amazonia will also be very slow due to logistic and climate problems for any wood-be invader. In this case the defender gets an advantage of +5 and the attacker gets a disadvantage of -6
Urban warfare also occurs in big cities of over five million people like OTL 2012 Kinshasa, Tianjin, Mexico City, London, New York, etc- Attacker -2, Defender +3
Attacks on major cities count as their own battle and need their own algo. Nation capitals (like Washington DC, London, Lima, etc), and cities over 15 million population (like Shanghai, Tokyo, Beijing, etc ), or historically major cities like medieval Beijing and Constantinople.
Storming on to a coastline from the sea or on to an island (first round only, so as your troops make a beachhead landing), the attacker -4 and defender +5.
Victim nations' life or death = +10 to the nation who is fearing being wiped from history, +5 if facing losing over 10% of its territory.
War Weariness: If you have been fighting for more than three turns in a row -5 due to battle fatigue until you make peace for at least two turns.
Fresh Military: If the army attacking or being attacked is new, +2
-10 In face of larger army,
-15 for completely surrounded,
+5 for in face of a smaller army,
+10 for surrounding enemy.
If defeated last turn, then the looser from that battle is -1 to his opponent in this battle.
Control of the Seas: 3+ attackers, or +5 defenders. (The nation with a superior navy in that area, that has control of the seas of the areas, has a major advantage, due to blockading ports, or cutting off the enemy's supplies.
Armed forces leadership (Depending on who is leading the army, if he historically successful leader like FldM. Rommel, Attila the Hun, etc.) Attacker: +1 Defender: +1
Blitz: Only available if the battle is the first of the war, or if the army is entirely light/medium tanks with air support. -4 Defender, +4 Attacker
Heavily bombed: Only available if the defending positions have been bombed for the past three months. -3 Defender, +3 Attacker
Scouts: If the attacker has sent scouts ahead (10% of the entire force, if the player declares them scouts) to scout enemy positions. Scouts may not participate in the overall fighting. -2 Defender, +2 Attacker.
Guerrilla Tactics (if you have no aircraft or armor in play): +3
Support from the Populace
Democratic Government supported by People +4
Non-democratic Government supported by People +2
Non-democratic Government unsupported by People -2
Democratic Government unsupported by people -4
War unsupported by People -6
War with a unrelated civilization prior to 1900 (Ottomans Vs Chinese, Rome Vs Genghis Khan, etc) Attacker -5.
Side under a blockade -3
Side under sanctions by a major power (Prussia, USA, USSR, France, etc) -10
Side under sanctions by middle power (Portugal, Spain, Ottomans, Turkey, etc) -5
Side under sanctions by a minor power (Montenegro, Zululand, Bolivia, Denmark, etc) -2
The player did not upgraded the infrastructure during the last 50 years (no new railways, factories, mines, roads, power stations, docks, etc) -1
The player did not build any supply or assault ships (applies only in attacking an island or coastline after 1910) -1
Add the totals up and find the biggest total. The biggest wins.