Aggression is a war-game for two players. Each has 100 armies to be distributed in certain countries on a map which is drawn at the start of the game. A series of battles are then fought with the object of neutralising enemy armies. The player retaining control of the most countries is the winner. There are three phases, in each of which the same player moves first.

Phase 1: The players take turns to delineate one country on the map. This is done by drawing the boundary lines of a single enclosed area on a piece of plain paper. There are no restrictions on the size, shape or location of the countries, though it is recommended that they be drawn reasonably large. Twenty countries are to be drawn in the standard size game.

Phase 2: The players take turns to allocate some of their armies to one unoccupied country. The number of armies allocated is written into the country concerned and that country is then regarded as occupied. The player taking first turn should identify his own allocations by boldly underlining his figures. Phase 2 continues until all countries are occupied or until both players have allocated all of their 100 armies.

Phase 3: The players now take turns to conquer one or other of the opponent's countries. A country may be conquered if there are more of your armies in adjacent countries than your opponent has in the country under attack. Similarly your opponent may conquer one of your countries, at his turn, if he has a superiority of armies in adjacent countries. To qualify as "adjacent", countries must have a visible length of common boundary. The number of armies in the conquered country is crossed out and they take no further part. The armies carrying out the attack are in no way depleted and may be used for other attacks later in the game. The object of the game is to retain more of your own occupied countries than the opponent retains of his. Note that countries you conquer don't contribute to your score but merely reduce your opponent's score by one as he will have retained one less country at the end of the game.

The best strategy depends, in complex ways, upon the original map. Several tips are:

Create "buffer" countries to protect weak countries during phase 2.

Avoid crowding enemy-held countries at the start of phase 3.

The first countries to attack in phase 3 are not necessarily the strongest of the enemy-held countries, but those which, by teamwork, can most damage your position.

—Eric Solomon, Games with Pencil and Paper