The aerial counterpart to Subs, Stealths are much more threatening do their much higher degree of versatility.
Stealths are the most versatile attackers in the game; the only target a Stealth is not capable of hitting is a submerged Sub. In the process, they can deal relatively high amounts of damage to most units it can attack, although it will do minimal damage to Mega Tanks. Attacking Anti-Airs, Fighters or Cruisers is also ill advised, as even on counterattack, these units will generally come out ahead of the Stealth in those engagements.
The Stealth's trademark ability is in its Hide command. When told to hide, the Stealth will become invisible to the enemy, much like submerging a Sub, only to be seen if the Stealth is adjacent to an enemy unit. The other benefit of hiding is that a Hidden Stealth may only be attacked by other Stealths and Fighters; no other unit may attack it, even on counterattacks. As such, Stealths can make excellent harassment units, attacking certain areas that an opponent cannot feasibly react to. If an enemy unit tries to move to a square a Hidden Stealth is on, they will be ambushed, and that unit's turn will immediately end without it being able to take any other actions. Stealths can make excellent shielding units in a pinch due to this feature, thanks to its general difficulty of attacking.
The main drawback to hiding a Stealth is it will rapidly consume Fuel; a Hidden Stealth burns 8 fuel at the start of each day, which combined with movement costs and its low fuel capacity, will cause it to run out of fuel in just a few days if it remains hidden. Stealths will have to be kept near an Airport or an APC in order to remain combat-ready throughout the battle. When using APCs to refuel a stealth, one may move the APC that may make the opponent think that the Stealth would be in one particular area, when in reality is in another. Misleading your opponent with an APC can keep your opponent guessing on where the Stealth is.
A well-used Stealth is one of the most challenging units to combat. Most of the attacks a stealth will make will be while it is hidden, meaning only Fighters and Stealths may attack it. If you see an opponent build a Stealth, you must build your own stealth or a Fighter to counteract it as soon as possible, before it has a chance to Hide and harass your other units. When you have such a unit, keeping all of your other units within range of it will restrict the Stealth's attack options.
Even without moving much, a Hidden Stealth can only stay airborne for about five or six days before it reaches critically low fuel levels. Even if you do not know where a Stealth is, can be easy to factor in that it will likely be relatively near to an Airport or an APC. Keep those particular areas under watch, as those will be areas with higher overall Stealth activity. Destroying APCs where possible can be beneficial in the long run, as denying a Stealth easy fuel access can severely restrict its effective range when Hidden. When an opponent is using a Stealth that is not hidden, the game Log will tell you where it moves to before it Hides, so if one does Hide, be sure to check the log first to make sure of which general direction it headed in.