I haven't played enough to test out all the trait combos, or even all the traits, yet, so this is not meant to extoll aggressiveness at the expense of any other traits. That said, I'm quickly growing fond of the aggressive trait.
The Trait
Aggressive brings with it three bonuses: free Combat I promotion for melee and gunpowder units, half-price barracks and half-price drydocks. All three are, of course, geared toward helping you assemble the most feared war machine on your planet.
Half-priced barracks are a staple from those good ol' militaristic CivIII civs, and the benefits are pretty well known. The only difference being that now, instead of four-hp land units, they give four xp to any land unit produced, which is good for one promotion right out of the gates. They're really good in the early game, allowing you to quickly throw up a barracks and get some veteran units into the field. They're not as good conquered cities as they used to be, since the main point to getting them up quickly when you took a city in CivIII was having instant healing in a frontline town. They no longer heal instantly, so the only benefit in a newly-conquered town is being able to create veteran units where they're needed.
As the game goes later, the build price becomes less and less significant compared to city production, and this benefit becomes less and less significant. It's always handy for throwing With the xp limits on animals and barbarians, though, and the elimination of barracks' instant healing effect, I submit that the cheap early barracks isn't quite as powerful as in CivIII, but it's not a very significant decrease.
I'm a big fan of half-priced drydocks. Obviously, the utility of this is highly map-dependent, less on Pangea, more on Archipelago. Drydocks are key to a dominant navy. First, they halve build times of naval units, meaning even mediocre producing coastal cities can churn out transports and the occasional combat ship fairly quickly. Second, they act as naval barracks, giving four xp to ships. Combine this with the Pentagon, and each ship gets two promotions, enabling Navigation I (+1 movement) before it ever sees an enemy. Build West Point in a coastal city and each ship from that city can be at Navigation II (+2 total movement) before leaving port. Movement is often key to naval warfare, so my personal preference is to give nearly every ship Navigation I ASAP, the exception being a few collateral damage battleships. There are two downsides to drydocks. First, they give one unhealthiness. This is easily overcome, though, with all the ways Firaxis has given us to combata it. Second, they don't become available until steel, so this benefit only really applies to your "modern" navy.
The most important, in my eyes, is the free promotion. While mobility is always crucial in warfare, those melee and gunpowder footsloggers form the backbone of any well-rounded army. A non-aggressive melee unit built in a barracks city has the choice of Woodsman I, City Raider I or Combat I. That same unit, belonging to an aggressive civ, has the choice of Woodsman I, City Raider I (Guerilla I for gunpowder), Combat II, Medic I, Pinch, Shock and Cover. This allows much more tailoring of your military to suit your specific game and purposes. With the Pentagon, you can get that unit to Combat III, Medic II, City Raider II, Guerilla II or Woodsman II (NB: City Raider, Guerilla and Woodsman are included for completeness, as they don't rely on Combat I) as soon as it's built, or divert a promotion to any of the specialty areas, like Amphibious or Pinch. Even before the Pentagon is available, that 4/5 Combat II Swordsman is a single victory from being Amphibious, if you so choose.
In the middle game, when barbarians have been pretty much subdued, it lets you tailor your military for the next war without having to beat up on another civ in the meantime (though why you'd want to avoid that, I have no idea
). In short, it allows you a lot more choices earlier on in the unit's life, which means more effective and appropriate units for your situation, which translates into a much more effective fighting force in the aggregate.
The Key Wonders/Improvements
Improvements
Barracks - Only 30 hammers, provides a second free promotion and, by itself, leaves a unit just 1 xp shy of a third promotion.
Drydock - Only 60 hammers, cuts naval build times in half, which means you make up those hammers on the first transport you build, provides one free promotion.
National Wonders
Heroic Epic/West Point - Placed in the same city, mean that you're churning out units twice as fast (HE) and with twice as much starting experience (+4 from WP) as otherwise. When those are gunpowder/melee (most likely gunpowder due to timing of WP), they're able to hit Combat III right out of the gate, and are only 2 xp shy of a fourth promotion. If you have coal and iron, Ironworks in place of the HE has the same effect, though the double non-unit production of IW could be better spent outside a unit specializing city.
Red Cross - Not really key, as it only applies to one city, but provides free Medic I promotion, and makes for some interesting potential combinations with the NWs listed above.
Mt. Rushmore - Never hurts to cut war weariness by 25% when you've got such an impressive army that letting it go to waste would be a crime.
World Wonders
The Pentagon - The +2 xp for any unit effectively means an extra promotion from all those barracks and drydocks.
The Kremlin - Cuts rushing costs in half. If you're pressed for time, buy your way to that big army you need ASAP.
The Civics
Vassalage - Provides +2 xp for all new units. In case you haven't deduced yet, the crux of this trait is piling up promotions as quickly as possible in order to field the most effective war machine you can. The lower unit support in the early to mid-game is also a big boon.
Police State - Adds 25% to unit production across the board and decreases WW by 50%.
Theocracy - Provides +2 xp for all units produced in cities with your state religion. See Vassalage.
The Empires
Note: Unique Units listed with only their differences from base unit.
Aztec
Montezuma - Spiritual
Jaguar (Swordsman): 5 str, 10 ham, +25% jungle defense
Mysiticism, Hunting
French
Napolean - Industrious
Musketeer (Musketman): 2 mov
The Wheel, Agriculture
Greek
Alexander - Philosophical
Phalanx (Spearman): 5 str, +25% hills defense
Fishing, Hunting
Incan
Huayna Capac - Financial
Quechua (Warrior): +100% vs. archery units
Agriculture, Mysticism
Japanese
Tokugawa - Organized
Samurai (Maceman): 2 first strikes, can not build with copper (iron only)
Fishing, The Wheel
Mongolian
Genghis Khan - Expansive
Kublai Khan - Creative
Keshik (Horse Archer): 1 first strike, ignore terrain movement cost
Hunting, The Wheel
My sincere thanks and sympathy to anybody who got this far.
The Trait
Aggressive brings with it three bonuses: free Combat I promotion for melee and gunpowder units, half-price barracks and half-price drydocks. All three are, of course, geared toward helping you assemble the most feared war machine on your planet.
Half-priced barracks are a staple from those good ol' militaristic CivIII civs, and the benefits are pretty well known. The only difference being that now, instead of four-hp land units, they give four xp to any land unit produced, which is good for one promotion right out of the gates. They're really good in the early game, allowing you to quickly throw up a barracks and get some veteran units into the field. They're not as good conquered cities as they used to be, since the main point to getting them up quickly when you took a city in CivIII was having instant healing in a frontline town. They no longer heal instantly, so the only benefit in a newly-conquered town is being able to create veteran units where they're needed.
As the game goes later, the build price becomes less and less significant compared to city production, and this benefit becomes less and less significant. It's always handy for throwing With the xp limits on animals and barbarians, though, and the elimination of barracks' instant healing effect, I submit that the cheap early barracks isn't quite as powerful as in CivIII, but it's not a very significant decrease.
I'm a big fan of half-priced drydocks. Obviously, the utility of this is highly map-dependent, less on Pangea, more on Archipelago. Drydocks are key to a dominant navy. First, they halve build times of naval units, meaning even mediocre producing coastal cities can churn out transports and the occasional combat ship fairly quickly. Second, they act as naval barracks, giving four xp to ships. Combine this with the Pentagon, and each ship gets two promotions, enabling Navigation I (+1 movement) before it ever sees an enemy. Build West Point in a coastal city and each ship from that city can be at Navigation II (+2 total movement) before leaving port. Movement is often key to naval warfare, so my personal preference is to give nearly every ship Navigation I ASAP, the exception being a few collateral damage battleships. There are two downsides to drydocks. First, they give one unhealthiness. This is easily overcome, though, with all the ways Firaxis has given us to combata it. Second, they don't become available until steel, so this benefit only really applies to your "modern" navy.
The most important, in my eyes, is the free promotion. While mobility is always crucial in warfare, those melee and gunpowder footsloggers form the backbone of any well-rounded army. A non-aggressive melee unit built in a barracks city has the choice of Woodsman I, City Raider I or Combat I. That same unit, belonging to an aggressive civ, has the choice of Woodsman I, City Raider I (Guerilla I for gunpowder), Combat II, Medic I, Pinch, Shock and Cover. This allows much more tailoring of your military to suit your specific game and purposes. With the Pentagon, you can get that unit to Combat III, Medic II, City Raider II, Guerilla II or Woodsman II (NB: City Raider, Guerilla and Woodsman are included for completeness, as they don't rely on Combat I) as soon as it's built, or divert a promotion to any of the specialty areas, like Amphibious or Pinch. Even before the Pentagon is available, that 4/5 Combat II Swordsman is a single victory from being Amphibious, if you so choose.
In the middle game, when barbarians have been pretty much subdued, it lets you tailor your military for the next war without having to beat up on another civ in the meantime (though why you'd want to avoid that, I have no idea

The Key Wonders/Improvements
Improvements
Barracks - Only 30 hammers, provides a second free promotion and, by itself, leaves a unit just 1 xp shy of a third promotion.
Drydock - Only 60 hammers, cuts naval build times in half, which means you make up those hammers on the first transport you build, provides one free promotion.
National Wonders
Heroic Epic/West Point - Placed in the same city, mean that you're churning out units twice as fast (HE) and with twice as much starting experience (+4 from WP) as otherwise. When those are gunpowder/melee (most likely gunpowder due to timing of WP), they're able to hit Combat III right out of the gate, and are only 2 xp shy of a fourth promotion. If you have coal and iron, Ironworks in place of the HE has the same effect, though the double non-unit production of IW could be better spent outside a unit specializing city.
Red Cross - Not really key, as it only applies to one city, but provides free Medic I promotion, and makes for some interesting potential combinations with the NWs listed above.
Mt. Rushmore - Never hurts to cut war weariness by 25% when you've got such an impressive army that letting it go to waste would be a crime.
World Wonders
The Pentagon - The +2 xp for any unit effectively means an extra promotion from all those barracks and drydocks.
The Kremlin - Cuts rushing costs in half. If you're pressed for time, buy your way to that big army you need ASAP.
The Civics
Vassalage - Provides +2 xp for all new units. In case you haven't deduced yet, the crux of this trait is piling up promotions as quickly as possible in order to field the most effective war machine you can. The lower unit support in the early to mid-game is also a big boon.
Police State - Adds 25% to unit production across the board and decreases WW by 50%.
Theocracy - Provides +2 xp for all units produced in cities with your state religion. See Vassalage.
The Empires
Note: Unique Units listed with only their differences from base unit.
Aztec
Montezuma - Spiritual
Jaguar (Swordsman): 5 str, 10 ham, +25% jungle defense
Mysiticism, Hunting
French
Napolean - Industrious
Musketeer (Musketman): 2 mov
The Wheel, Agriculture
Greek
Alexander - Philosophical
Phalanx (Spearman): 5 str, +25% hills defense
Fishing, Hunting
Incan
Huayna Capac - Financial
Quechua (Warrior): +100% vs. archery units
Agriculture, Mysticism
Japanese
Tokugawa - Organized
Samurai (Maceman): 2 first strikes, can not build with copper (iron only)
Fishing, The Wheel
Mongolian
Genghis Khan - Expansive
Kublai Khan - Creative
Keshik (Horse Archer): 1 first strike, ignore terrain movement cost
Hunting, The Wheel
My sincere thanks and sympathy to anybody who got this far.

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