Aliens that "join" the Axis, but they have other plans... You don't spend resources travelling from one star system to another just to help out some technological backward carbon based O2 breathing biped sentient beings win a stupid terrestrial war between each other, using up resources and potential slaves. They have transcended such petty conflicts where their focus is protecting their system(s) from other possibly hostile technologically advanced beings. What side they would join in a pre-Atomic terrestrial conflict doesn't make any sense.
Since they have the tech to come from another star system, they would probably have:
Defensive armor able to neutralize any threat our weapons have upon it like a force field or exotic metal alloy (Neutronium) and composite hull plating with ablative armor and adaptive self repairing properties
Weapons capable of destroying entire formations of units or whole cities with one shot (fusion weapons, plasma weapons, particle beams, electrolasers, antimatter warheads, etc)
Weapons capable of incapacitating or paralyzing through non-lethal means either by restraints, gas, electroshock, flash, high pitched sound waves, radiation, etc.
Weapons capable of rendering anything electric useless (EMP, ion cannon)
Jammers that can jam any type of over the air and wire communication and sensors
Tractor and repulsor beams capable of attracting or repulsing objects of certain mass depending on power of beams
Why take sides in a war you could end in a matter of days? If they did potentially see a benefit from the Axis in charge of Earth they could do many things with the technology they have. Lets see, if they were slaver aliens they would use some of their weapons to destroy or capture all military hardware. Then destroy all facilities used to make them. Then destroy all facilities used to make raw materials for military hardware. The Allies would then have no ships, planes, vehicles or weapons to use nor the facilities to make them. They could go one step further and capture, kill or render useless the people necessary to build any weapons and facilities capable of doing so. Then destroy any knowledge available on how to do so. After they do that what use is the Axis other than puppets? What if Hitler doesn't agree with them? They could easily replace him but without Hitler can the Axis be?
If they were genocidal aliens who only want Earth for colonization or resources they could sample all biological infectious diseases that can kill humans and make a cocktail of them that has easy quick transmission between humans and guarantees death. Infect the most populous places first, maybe put it in water and food supplies elsewhere or they could simply inject it in every human. Another way may be to simply sterilize humans and wait until present generations die off and come back later and take over. This kind of takeover may take longer but infrastructure will stay relatively intact.
Then you might have aliens that genuinely need us and see our potential. They would end the war between us and find a way to show us how pointless war between each other is, if we haven't already figured that out after discovering we are not alone. They could gather out greatest minds and transfer to us their knowledge to learn and spread in stages. First obstacle would be communication. Then learning their math. Once that is understood we could continue. Then they could help provide the basics; running water, food, electricity and data (worldwide text, voice and video communication) to every human.
Connect all places of the world with clean high subsonic maglev trains (~765 mph) and hypersonic aircraft. All land based vehicles can comfortably travel at 260 mph, and have acceleration and braking of a F1 car
. All road infrastructure is built to handle large cargo land vehicles that can travel that speed. When parked, all land vehicles are able to fully charge in the time it takes to fuel up a 16 gallon fuel tank and have an electric range of over 2000 miles.
Automate factories and laborious work with robots. Eliminate all dirty and inefficient fuels and power sources. Teach everyone on Earth a common language and adopt a planetary currency. Work towards a true world government, but allow regional autonomy. Provide basic emergency and health services for every citizen. Free Kindergarten through Doctorate education. Citizens' specialties based on skills and intellect. There will be no shortage of work in this new Post-Modern Space Age!
A world military based on alien tech. Soldiers, or Marines rather (considering the navy will transition to airships), will be able to hop on a hypersonic aircraft and get to any part of Earth in an hour or less. To get them down stealthily the aircraft slows down to Mach 3 and the Marines, get shot out at high altitude with special suits able to take the G's, pressure, cold and navigate them on target all the while gathering intel on adversary and drop zone and then open their chutes at safe minimum calculated elevation that their suits can withstand (not bodies). Along with them are anti-gravity vehicles able to carry huge loads across any terrain. They are armored sufficiently to protect them from any known man portable weapon and have enough countermeasures to get them out of range or confuse dedicated anti-land vehicle weapons. The weapons on this air dropped vehicle are able to take out land based tanks and strike aircraft. The roles of tanks and strike aircraft have merged now that the navy is airborne but there still exist land based tanks that look similar to pre-alien tanks but have significantly upgraded systems.
With this alliance with this alien race comes a deal that we support them. At first we never had a choice and later will come to realize that we could break the deal. The aliens anticipate this either through their own history or ours and thats why they don't divulge all information to us. It's also why they have substantial resources towards detecting other alien races and preventing them from making their own deals with us and helping us overthrow our overlords. When the time comes for humans to revolt the first time we learn a little of the alien's true power. They don't even use their full might. We surprise them for the first minutes but within an hour the Terran Federation military is destroyed. Sleeper agents are embedded in the military and activate as soon as news reaches alien governor. They already knew our plans but not when, so when they did all they had to do was activate agents (humans chosen to get implants that when activated cause them to follow an alien script) that conducted assassinations, sabotaged vital command and control centers, disabled key weapons, inputted erroneous data, you name it. This cycle of revolution and defeat would continue until eventually the humans could match an expeditionary alien force sent to destroy us again. The aliens allow us to rebuild for that is their plan all along. We get smarter and more cunning, making us bolder. The aliens knew our nature and are only nurturing whats innate in us. Even though small and vulnerable we have an aggressive spirit. Truth is the aliens have run into a larger more powerful alien race and need help. They have watched us progress until we almost wiped each other out and then they stepped in and stopped it. With the hope of exploring the stars we were able to see beyond our planet and sun and see the possibilities. A human civilization spread across the universe! Moving from the Terran Federation to the United Federation of Planets to project the International Fleet or Starfleet "to explore strange new worlds; to seek out new life and new civilisations; to boldly go where no man has gone before."