TripleA is an open-source turn based strategy game and java-based board game engine.

The gameplay is based on the board game called "Axis & Allies", and TripleA allows you to play by these rules, or similar mechanics, on many old, new, and custom maps, both for online multiplayer, or play-by-email, or single player vs AI or hotseat mode. It comes with 11 different maps that can be played, and the option to download more from online repositories.

Generally most games take place during World War 2, though there are mods to play other conflicts like the Napoleonic wars, or Lord of the Rings. The players are divided up into the two opposing sides of this conflict, and each takes control of one or more of the nations involved.

Though some games have custom rules, most follow this pattern:

Each nation takes a turn, in a predefined order. At the beginning of a turn, that nation must choose what to purchase with the resources it gathered during its last turn. They spend this money on new units, like soldiers, ships, and aircraft, or on technology that could improve their current units. These units are set aside and will be put on the 'board' at the end of the nation's turn.

After purchasing is done, the nation may move any of its units to engage in combat. The player must make all combat related moves now, as combat only occurs once per turn (unlike the similar boardgame, "Risk").

When all moves are completed, all battles are resolved in an order the attacker chooses, by rolling dice. First the attacking nation throws one dice for each unit they are attacking with, then the defending nation remove any of their units killed then throws one dice for each defending unit, with the attacking nation choose which of his/her units will die. Each unit has an attack power, and defense power, so for each dice thrown the number on the die must be equal to or less than that unit's power for that unit to "hit" the enemy, while any number larger is considered a "miss".

After all battles finish, the nation may move any units that have not yet moved, and any aircraft must return to land in territories owned at the beginning of the turn.

At the end of the turn, the nation places the units they bought at the beginning of the turn, and then collects income equal to the combined value of all of the territories under its control. The next nation then begins their turn.
Games last until certain objectives are met, or one side surrenders.

Minimum system requirements: 

A java runtime environment