Be warned that this guide is for those having problems on Ensign i.e. the lowest difficulty. Or at least, having problems with getting all the secondaries.
1. Always elite repair for core units and normal for non-core.
2. I will never play Japanese.
My opinion of WWII Japan begins with "Should have blockaded them until tens of millions starved/froze to death over the winter of '46 instead of nuking some sense into them." and goes up in retribution from there. Imperial Japan did not surrender. "The war situation has not developed necessarily to Japan's advantage" is not "We are utterly beaten and throw ourselves at the mercy of the victorious conquerors" and therefore is not good enough...
And before anyone whines about subtlety, if the Japanese language is incapable of communicating such basic concepts to someone who has an elementary grasp of it then it is not a method of communication. And if they cannot communicate their surrender, then their destruction will continue.
Hirohito should have been hanged like a dog for all his gas and biological attacks in Asia. And if that means the Japanese collectively lose their minds even further, then Nihonjin Delenda Est.
My opinion on modern Japan on the other hand, besides the war crimes denialists (Osaka's mayor ended its twin city relationship with IIRC San Francisco in 2017 over a comfort woman statue) who still rate a DEUS VULT, is "great anime, mangas, and most importantly, doujins", aka DESU VULT.
3. I am almost certainly never going to play Germans unless I feel like raving about how denazification demonstrates the true glory and righteousness that cultural genocide can embody (because that's what denazification was, and good riddance!).
US Pacific Mission 1: Pearl Harbour
1st turn: Park your DD near the BBs early on either north of the eastern one, or south of it, that's some AA support from that boat.
In the first turn go to the town next to the harbour with your medical jeep. Then use that AA unit to activate the southern airfield.
2nd turn: Move that first AA unit near the western two battleships, notice the center battleship is on a Port grid. Send the jeep running off to the eastern airstrip.
Notes: First priority for fighters is to maim inbound Kanko units i.e. get them before they drop torpedoes. It's very possible to keep all 3 battleships alive, even though AA units will mostly be running around the map until very late.
Notes: Do not park your fighters over top of battleships. Somehow the IJN bombers can muscle them out of the way. Park them in a shield across the tops of the BBs.
3rd turn: Your AA is now a boat right next to Ford Island's airfield. The jeep keeps running. When the B-17 shows up move it somewhere north of Peark Harbour to maintain continual radar coverage on enemy exit vector. But you don't have the fighters to pursue yet usually.
4th turn: Your AA has activated the Ford Island airfield. Jeep continues.
5th turn: Your AA is re-embarked because units can do that immediately after landing.
6th turn: Your 1st AA is sailing west and is next to a bit of forest next to the road.
...I forget when the wildcat from Enterprise shows up. But when it does, send it to join the air screen.
Your 1st AA should run all the way down to the southwest airfield and village. The AA from that village comes with a truck and should be sent to the north to get that airfield and village, then onto the northwest corner village.
Once your jeep gets to the eastern airfield, make it run to the eastern villages. Unlike the AA units packed into trucks, the jeep can go over difficult terrain.
US Pacific Mission 2: Philippines
Make sure you spend 5 Spec points on Infantry Training and flood the field with US Infantry '41s. However do not begin by putting any on river grids at the top if you can help it. MAYBE trade two of those for your Australian Commandos and a Stuart, and get as many heavy infantry as you can without losing any units, and you'll be just fine.
If you really want to troll around, buy a wildcat and put that and the P40 from the first mission on the western airfield. Gang up on the Japanese planes and you'll eventually beat them back.
Philippine Recruits aren't great but they can finish off enemies after your main units batter them to low HP. It saves your main units' attacks. It's actually quite possible to occupy all the initial Japanese supply points and keep beating off their reinforcements until the very last wave which comes in the sides quite far down the map. If you particularly want to, you can probably beat that wave off too if you keep track of the turn the last top-entry wave comes in (somewhere around turn 10-11 IIIRC), massacre those in one stroke, and then march south ASAP.
If you're concerned, you can just hold the line at the top rivers (where the initial line is) and station some Philippine Recruits in the northernmost line of secondary VPs (silver spear tip on the flagpoles, you may have to zoom in) as insurance. However, this would let the Japanese build up to become more overwhelming and likely actually force you to fall back over time.
US Pacific Mission 3: Bataan
I for one got War Economy Spec. It was basically irrelevant.
The hard part is knowing to retreat on Turn 5 after nabbing that 2ndary, as long as you're not at risk of losing any 2ndary VP i.e. enemey is still bogged down on river away from the VPs OR on other side of unbridged rivers. Your PT boats should also be hovering around the middle of the west side of the map waiting for the attempted landings to torpedo the supply ships, then shoot up the transports. If you don't fall back quickly enough... or know to save scum if the accursed fortress RNG isn't cooperating, you won't get the 10 damage from the fortress gun.
Take free advantage of the discounted upgrade to US Infantry '42 and hold the line just south of/around the fortress in the east, and the bunker-side town in the west (the 2nd enemy landing comes south of that). On Ensign, the 2ndaries are really pretty easy.
If you feel uneasy, use your Philippine Recruits to snooze in the primary VPs (gold star on staff).
US Pacific Mission 4: Marshall / Gilberts Raids
Get 2 radar upgrades for Specs.
Buy a Lex, to save some money, dump it and your PT boat as far southwest as possible, then pop in a wildcat (could be from mission 2), an SBD, and a Devastator (carrying bombs). Later in the match buy another Lexington as soon as you can afford, and Devastators too unless you aren't as cheap as I am. Move your fleet southwest and attack the middle island, then south, then go northwest. The PT boat is great at shooting up the things you'll find on land in this match except perhaps the shore guns and the Japanese engineers.
Notes: The's a shore gun in the middle island and another on the island south of the westernmost airbase. These and planes are the main dangers to your carriers. Blow them up with your bombers and if need be your ships.
You can ignore the enemy AA guns and just focus on their planes, ships and fuel tanks.
The 3 fuel tanks are on the south, north, and northwest islands. I for one spotted no submarines even though my carriers and support ship were blundering around on their own for most of the mission.