Information/Atomic Era Guide and Tips

Before using nuclear weapons, you must make sure you have researched the "nuclear fission" tech. After doing so, you will unlock the manhattan project. Once completed, you have the option to build a nuclear device in the menu when choosing production. If you want to build a larger nuke, you can research nuclear fusion. Doing this will unlock operation Ivy. This will allow production of a thermonuclear device, a much more powerful nuclear bomb. After this, you may use a nuclear submarine, bomber, jet bomber or nuclear silo to launch a nuke. You can do so by clicking "WMD Strike". You can see how many nukes you have in the top right corner of the screen.
You should be careful when launching any nuclear device. It is not always a good idea to launch a nuclear strike directly on the city center. The nuke will destroy all adjacent tiles, and the thermonuclear will destroy even more. Both will contaminate all tiles it destroyed, though the thermonuclear has a longer lasting effect. All contamination can be cleaned up by builders, and any unit on a contaminated tile will lose 50% HP per turn. When invading, you should launch them on the tile that will destroy the least amount of specialty districts. You should also check to make sure that the target city doesn't have a mobile SAM or destroyer on the city center or on an adjacent tiles. These will shoot down the nuclear device.

Defending from Nuclear Attacks

Defending from attacks are very important. Do not be fooled by the bots, because they will use nukes as well. I recommend keeping a Mobile SAM at all city centers. If the city is on the coast, a missile cruiser or destroyer will work. These will destroy the nuke before it makes it to your city. They will also help defend against air attacks on any surrounding tiles. Just remember, other civilization can also do this, so check before bombing a city.

When declaring war to conquer another player's city, make sure you are a declared friend to as many civilzations as possible. Also, always declare formal war with casus belli if possible to minimize warmonger penalties. Another option is a joint-war. Even though warmonger penalties are increased, it can be helpful to have someone fighting alongside you.

Tactics of War

The best weapon by far to use for war is air attacks. Bombers and extremely good for pillaging and nuclear attacks, as well as defortifying a city. Fighters are more useful for closer land unit or naval attacks. The weak spot for all airplanes is anti-air class units. Naval unit can also be useful. The missile cruiser is good for defending against air attacks, or destroyers which deal much damage. Both air and naval units are good for destroying melee or ranged units in which are embarked in the ocean. Embarked land units cannot defend and are vulnerable to any type of attack.

It can also be good to keep a machine gun or rocket artillery at a city center on top of the protection received from walls to fight nearby enemies.

One of the absolute best ways to invade a city is to declare war, nuke the city, and conquer it on the same turn. You can do this by setting a melee unit on the border of yours or an allied country and the target city, and making sure you have a unit in range to launch a nuke on the target city with, and the melee unit can reach this city on the same turn it is nuked. If the target city is a coastal city, a destroyer will work and may be better because it can move more units in a turn on water.

Gold can be essential for trading and purchasing immediate units. Units also cost more to maintain later in the game So it is a good idea to keep a strong income. One of the highest incomes for gold is trade routes. Always keep as many as possible to prevent you from going into debt. Another way to prevent going into debt is to use some of the following government policies.

Commercial Hubs

Commercial hubs provide a good income. It also unlocks the project, "Commercial Hub Investment", which provides a lot of gold every turn. I recommend this being the first thing you construct after establishing a city.

Trading

During a war, you always try hard to be winning as dominantly as possible before you end a war. Take as much as possible. Depending on how much you take or how serious of a threat you are the the enemy nation, they will be willing to give up a lot if they have nothing left to fight with or have lost a lot. Siphon as much gold as they are willing to give, which is usually most of it, if not all. It is funny watching the nation give you all their income for 30 turns while they are already struggling to rebuild their empire that you wrecked.

When you are late in the game, you will be able to compete in the space race. If you don't already know what this is, it is a series of researches that you can do to launch different rockets into space. A space station is required. If you manage to complete and launch all the space shuttles before anyone else, you win a science victory.

Satellites

The most useful space race project is, "launch earth satellite." This reveals the entire map. It can also be useful to be an ally of a civilization who has already completed the project.

Spies

Sometimes, when you build a space station, you will find that it sometimes gets pillaged when no visible enemy can be seen. This is because other civilizations (including NPCs) will send a spy to disrupt rocketry. It doesn't tell you where the spy is. One way that will help is to counterspy on the space station. It can also be useful to send spies to disrupt rocketry to other civilizations. You do not have to be at war to send spies.

If you made it this far, thank you for reading.

I may add to this in the future, so please comment if there is anything you think that I missed.