Envelopment

Arguably the most important tactic a general must know, this allows swift, absolute and total annihilation of a certain portion of the frontline, as opposed to just pushing it back. It's only requirements are a slight superiority in detail. Say, an enemy can have 100+ divisions overall, but if the player has 20 and the right conditions, he can use this tactic repeatedly until the enemy is totally destroyed, or until you run out of motorized divisions.

There are, of course, variations, of which some involve using a fighter squadron on an adjacent empty (your) tile, paratroopers behind enemy lines or a fleet blocking the naval escape path.

Pros

 * Impressive destructive potential, game-changing maneuver.
 * Economical, preserves backbone divisions very well.
 * Does not require an airforce, and most often not even a navy.

Cons

 * Causes attrition for breakthrough-capable divisions (Motorized and Armored), which are fairly expensive.
 * If enemy army is stationed in a province surrounded by too many neighbouring provinces (8+), this tactic might be impossible or prohibitively expensive to pull off.