History GameBreaker / HeartsOfIron

** An "engineering" light tank company, which consists of a mere 15 light tanks fitted with bulldozer blades, extra armor skirts [[TheLastOfTheseIsNotLikeTheOthers and flamethrowers]]. It offers both attack and defense bonuses in all sorts of terrain, along with movement speed bonus and when combined with regular engineers company create a combo of dirty cheap[[note]]In fact, an engineering tank company is ''cheaper to field than regular engineers'' and definitely less resource-intensive[[/note]] power multipliers that allow to defend any spot on the world and also operate in the harshest terrains as if it was a walk in a park. Truly, they clear the way.

** Both ''III'' and ''IV'' have similar game-breaking strategy when it comes to expanding the US war industry. In ''III'', the US could build handy infrastructure, gain Construction Practical Knowledge while doing so, and then just spam factories with a heavy discount, using their pre-existing industry to do it at massive scale. In ''IV'', US starts with bunch of debuffs to construction... except for infrastructure. And high infrastructure increases both the resources of that state ''and'' speeds up the construction of all buildings, including factories. So until about summer 1937, players behind the US can completely trivalise the rest of the game simply by building roads and such.

** [=By Blood Alone=] DLC introduced an ObviousRulePatch that made it impossible to arm a heavy cruiser to the specifics needed for a meta... but made ''battlecruisers'' the 1:1 replacement: slightly more expensive, but way deadier than [=CAs=] were in their [=pre-BBA=] form, and also far more survivable. The only thing that patch caused was unintentionally making deck conversions into auxiliary carriers less useful, since battlecruisers are now better refitted with different guns than rebuild as stop-gap carriers (which is still viable strategy, just leaving the navy with less suitable candidates for the conversion).

* With the post-[=MtG=] overhaul of naval combat, [[InNameOnly "heavy"]] cruisers backed by swarms of the cheapest destroyers became '''the''' way to handle navies. Forget all the {{Cool Ship}}s, they are just useless sinks of resources and dockyard capacity and will be sent to the bottom in zero time. The heavy cruiser design fully exploits how targets are acquired and how damage is distributed. Thus, by making a cruiser ship with the cheapest and weakest heavy gun battery and then filling everything ''else'' with light guns that deal damage to screens, creates a floating gun platform that will decimate enemy screens with impunity. Forget armour, AA guns or "big" guns, they are utterly redundant - all that matters is maxed-out light damage output and speed, with that one token battery of heavy guns just to qualify as a capital ship. Meanwhile, your own screens can be decimated themselves, but as long as just ''one'' survives, the moment the heavy cruisers remove enemy screens, the torpedoes will send to the bottom all enemy capital ships, since nothing is protecting them anymore. All of this gets put on overdrive when using the naval designer and doctrines decreasing visibility (and those doctrines improve cruiser perfomance, too). '''Any''' naval engagement where one side isn't following this meta will lead to losing ''the entire fleet'', and in multiplayer, the side with bigger stack of ships will endure due to sheer numbers. On top of it all, the resulting ships are ''very'' cheap and easy to replace, meaning whatever losses the navy will suffer can be replaced within a few months, while repairs of damaged units are a matter of just a few weeks or even ''days'', rather than months due to their default cheapness.