Engineers are potentially the most useful infantry types for all factions. While they have inferior combat statistics to Commandos or Heavy Infantry, they can deploy traps for enemy forces to delay them.
To utilize this ability, press TAB to bring up a menu with options to deploy mines, barbed wire, or Dragon's Teeth. Clicking on one of these options deploys the selected trap on the hex the Engineer is on.
These are useful for varied situations, such as bolstering a defensive force cheaply or blocking a narrow pass.
For U.S. factions, they are known as Engineers.
For the European Union, they are known as Sappers.
For Russia, they are known as Pioneers.