Ignore "Jet Turbine Development"; this wastes a great deal of resources for very little in performance over piston-engine models. Instead, rebuild your Attack Aircraft and Dive-Bombers with greater range and defense. This will greatly
improve their damage to enemy targets, but it will make them slower. Make adjustments to your Dive-Bomber/Attack Aircraft mix on aircraft carriers before attacking. The Dive-Bombers are far more effective against airfields than Attack Aircraft. Remember, it takes one week before the new planes will arrive and are useable in a strike.
Fleet management
Always keep at least one Fleet open for new ships at the Home Port. Individual warships cannot be sent off to join a Fleet alone, and without an open "slot", they remain in the Shipyard, not in the war.
Heavy Cruiser class
As any nation, immediately rebuild your best Heavy Cruiser class to include as much torpedo armament as possible. As soon as you receive "Middle Diesel Engine 1", upgrade that Heavy Cruiser with a higher Torpedo armament. The best
you can get is "Quintuple" with "10 Bay". These will prove devastating in ship vs. ship battles while retaining their bombardment abilities.
Intelligence Operatives
Try to keep an active "Intelligence Operative" in each enemy Home Port. You will at least know what they are building, what they are holding onto in their Shipyards, and how they stand on the Investment Scale for New Weapons.
More room for weapons and armor
Reducing "Speed" by even 2 knots on a modified warship design yields a lot of room to add more powerful weapons and/or armor. This is especially true for CA's and CL's. By stepping them down to 30 knots and reducing "Scout Planes" to 2 creates additional capacity for armament with very little loss of battle performance, but a far deadlier ship in combat.
Prevent night attacks
If you do not like the enemy attacking at night, continue to return to the title screen from the options menu. Eventually, the enemy will attack in daylight.
Stealing ship designs
Do not hesitate to "steal" other nation's new ships that are available to build. Look for the highest steel limit in a class (for example, BB's Yamato-class), and copy it to your navy for development. By selecting the one with the highest displacement to steal and use, there is greater room for additional armament, aircraft, or armor. If you are not playing the U.S. and are into carriersm wait for them to develop the "Midway-class" and steal the design. The same thing also works with aircraft.
Submarines
The only country that submarine warfare is effective against (killing transports) is Japan; who is desperately short of both steel and oil. Submarine attacks as fleets also lose their effect once the war goes past one year (Sonar, Hedgehog, etc., makes life ugly). If the enemy (especially Germany) attacks with solely Submarines, retreat your Fleets. Submarines cannot hurt airfields, and the game's AI will give them a kill or two if you hang around; especially against your capital ships (carriers and battleships).
Winning as the Americans
When playing as the Americans you will need the following to defeat other fleets: fifteen battleships and only a few aircraft carriers. Also, only attack at night.
