Few decisions shape an entire game like your land combat doctrine, because switching your grand doctrine later wipes all the mastery you've built up across all four tracks. The 4 options, with their real bonuses, plus the detail almost no guide explains properly: subdoctrines are separate and don't depend on which one you pick.
Mobile Warfare
+10% division speed and +20% planning speed, and unlocks the "Unexpected Thrust" tactic. The classic Blitzkrieg: tanks, motorized units and exploitation speed to break the front and encircle the enemy in pockets. It needs an industry that can actually sustain armor production, so if your economy is stretched thin, this isn't your doctrine.
Superior Firepower
+10% soft attack for line artillery and +5% for support companies, unlocking "Overwhelming Fire." Artillery, entrenchment, and a defense that holds no matter what's thrown at it. Fits wars of attrition and nations that would rather hold the front than break it. Less flashy than Mobile Warfare, but brutally effective on defense.
Grand Battle Plan
+10% max planning and +0.25 daily command power gain, unlocking "Well-Planned Attack." The most balanced and flexible option: planning offensives through the general staff, with bonuses that don't lean as hard on any one unit type. A solid pick if you don't want to lock into a specific style of war from minute one.
Mass Assault
−10% to non-combat out-of-supply penalties, and +48 hours of supply grace, unlocking "Relentless Assault." For nations swimming in manpower with no intention of relying on expensive gear. It's the natural doctrine for the USSR or China, where troop volume is the real edge, not sophistication.
Subdoctrines Are Separate
Here's what almost nothing explains properly: your grand doctrine (one of the 4 above) is only part of the tree. Underneath it sit four independent doctrine tracks — Infantry, Artillery & Combat Support, Armor, and Operations — and in each one you pick your own subdoctrine, unrelated to which of the 4 grand doctrines you chose above. In other words: you can run Mass Assault as your grand doctrine and still pick whichever armor subdoctrine actually suits you.
Here's what the tree looks like from the inside: four independent columns (Infantry, Combat Support & Artillery, Armor, Operations), each with its own mastery tiers (I through V) and, on completing it, a choice among several subdoctrines — in this example, the Armor track offers Armored Spearhead, Armored Infantry Support, Streamlined Deployment, or Mobile Defense.
Each track accumulates "mastery" independently whenever certain units see action (or at 10% of that rate while merely training), and every 100 points of accumulated mastery unlocks a new tier of bonuses within your chosen subdoctrine. The four tracks don't build mastery at the same rate: Artillery & Combat Support gains it at 10 times the base rate, Armor at 4 times, Infantry at the normal rate, and Operations at 70% of that rate. If you haven't picked a subdoctrine in a given track yet, mastery still banks in reserve (up to 200 points, at 25% of the normal gain) for whenever you do activate one.
How to Choose Yours
Ask what you've got plenty of: industry and a need for speed, Mobile Warfare; you'd rather take the hit before answering it, Superior Firepower; manpower to spare, Mass Assault; not ready to commit, Grand Battle Plan. Each track's subdoctrine can be adjusted later with a lot more freedom, since it doesn't depend on this initial choice.
If you'd rather not work it out yourself, this site's nation guides already recommend the most viable doctrine based on the resources that specific country starts with.
Where Value Gets Lost
The costliest mistake: switching your grand doctrine mid-game without knowing it resets all the mastery accumulated across all four tracks, not just the grand doctrine itself. If you've spent months building up mastery in Armor or Artillery, switching from Mobile Warfare to Mass Assault wipes it clean. Better to get your grand doctrine right early — based on what your industry and manpower already give you — than to fix it late and lose that progress.