Risk is a multiplayer game, which is opposed by both human and computer players.
In general, the basic strategy is as follows:
Use a single large group for attacking, as opposed to several smaller groups. This allows chaining attacks as far as possible.
During reinforcement, try to have twice the number of attacking armies compared to the armies found in the territories being conquered.
Focus on getting and holding continents first. Ideally, Australia serves as the best starting continent, with South America being second and North America being third.
If playing with fixed card exchange, switch the cards as soon as you can. If playing with incremental rules, it may help to delay until you get a full set of five cards.
If forced onto the defense, it's slightly better to have an even number of armies on the border.
The AI players tend to make mistakes when they would have otherwise won or gain a significant advantage.
The board game normally allows informal alliances and deals that can last for some time. While they may work in the computer adaptation, AI players only think about themselves and never cooperate. As such, you can treat them as those that attack random rabble.
Continents give a significant army, but they can be hard to hold. If you can't hold a continent, instead focus on claiming one territory at a time, or by trying to prevent other players form holding their continent. If your army stack becomes large enough, you can then claim a continent, and still have territories to spare.
