Invasion is a war game... and obviously in a war game the player needs Units/Troops to complete specific task that they are needed or assigned to do. Inside of the Invasion universe it could be either for Occupying, Reinforcing, Attacking or Gathering Resources. This page will show you what are the ups and downs of each unit available, from the weakest (Tier 1) to the strongest (Tier 4).
In Invasion You cant really just create a bunch of the same units mostly because of the "Unit Strength Trinity". There is really only 3 main units in the game. The "Cannon" units are classified as a "Artillery" unit and can only target Defenses the best. They are weak when going against SUV's, Tanks, and Copters. Although the Cannons are the " weakest" each unit of them gives out more Power, has a bigger Load which makes it the unit to gather the most resources and occupies Tiles the fastest.
The way the Trinity works is...
Tanks are stronger than SUV's; SUV type units are stronger than Copters; and Copter type units beat Tanks. When a tank goes against a tank... well it mostly goes to what tier they both are in or if it's a tier 3 tank vs another tier 3, its a even fight... they both die.
As stated above, Cannon type units cause the most damage on Defenses. But are weak against the other 3 type of units.
Each Type of unit produced will come with a upkeep... you gotta feed your troops.
"Units will continue to consume food. When consumption exceeds food output, the color of food reserves quantity will turn red.
You can obtain food through many methods, such as: Increasing the number of farms, upgrading farms, Commander talent, gathering, purchasing, trading, looting, and resource application."
~in game "Notice" Description
A tier 1 unit will bring up just 1 upkeep, a tier 2 will bring 2, a 3 will bring a 3 and so on and so forth.
However, its important to mentioned that it is not required to keep a positive food production (food made) to food upkeep (food required for troops) ratio. As a player progresses in the game it becomes increasingly harder to keep this ratio positive. Having a negative ratio has no impact on the units of the player. It also doesn't prevent the player from producing more units, so long as they can acquire the food necessary to build them. It only makes it impossible to maintain reserves as it will eventually make its way to zero. To offset this, players will gather and use food only when they need produce something.
Its a common practice for players to build only one farm and use the additional space for other resource buildings.