Land combat doctrine is one of the decisions that shapes an entire game the most, because switching branches midway costs years of already-invested research. Here are the 4 options, straight up.
Mobile Warfare
The Blitzkrieg doctrine: prioritizes tanks, motorized units and exploitation speed to break the front and encircle the enemy in pockets. It demands an industry capable of sustaining armor production, so it's not the best choice if your economy is weak.
Superior Firepower
Centered on artillery, entrenchment and very solid defense. It's the doctrine that best fits wars of attrition and nations that prefer to hold the front rather than break it. Less flashy than Mobile Warfare, but very effective on defense.
Mass Assault
Built for nations with tons of manpower and little interest in relying on expensive equipment: bonuses to numerous infantry and manpower reserves. It's the natural doctrine for countries like the USSR or China, where troop volume is the real advantage.
Grand Battle Plan
The most balanced and flexible option, centered on planning offensives through the general staff and bonuses that don't depend as much on a specific unit type. A good choice if you don't want to fully commit to a particular style of war from the start.
How to Choose Yours
Ask yourself what you have plenty of: if it's industry and you want speed, Mobile Warfare; if you'd rather absorb the hit first, Superior Firepower; if you have manpower to spare, Mass Assault; if you don't want to commit yet, Grand Battle Plan. Switching doctrines later is possible, but you lose a good chunk of already-researched progress — better to get it right early than correct it late.