There are no set rules as to how you go about fighting your enemies.
Some players may choose to be helpful and charitable, aiding their neighbours in any conflict, while other players will simply attack any army that comes near. Some players may be honorable and make steadfast alliances, while other may use more unscrupulous methods to catch your armies offguard.
The best way to enhance your combat strategies is to be familiar with the game-rules, and keep in mind that all player tactics may vary.