Countermeasures are defensive devices used by military aircraft to protect themselves from enemy missiles. Initially exclusive for enemy aircraft, they are usable by the player in Ace Combat: Assault Horizon and Ace Combat 7: Skies Unknown.
Aircraft countermeasures work by emitting a high heat output, equaling that of an aircraft engine, which cause heat-seeking missiles to confuse them for actual targets, thus leading them away from the aircraft that deployed them. The most frequently featured countermeasures in the Ace Combat series are the traditional defensive flares. Ace Combat: Joint Assault introduces the plasma-based Infrared Countermeasures as a special weapon.
Throughout the Ace Combat series, countermeasures have been mostly relegated to NPC use. For example, ace pilots, such as "Zipang" and "Decoder" as well as Yuktobanian B-2As from Ace Combat 5: The Unsung War, the F-16XL of the Wizard Team and the Grun Team from Ace Combat Zero: The Belkan War employ flares to spoof the player's missiles. In Ace Combat 6: Fires of Liberation, the Strigon Team uses flares and chaffs to spoof the player's missiles. Countermeasures are seen to be used by aces in Ace Combat: Assault Horizon Legacy too.
However, flares are finally made available and usable in Ace Combat: Assault Horizon, wherein they are deployed by clicking the left stick. All playable planes can use a small number of flares with very long reload time for balancing purposes. All fighter-type planes carry 5 flares, all multirole-type planes carry 6 flares, attacker-type planes carry 9 flares, bombers carry a supply of 99 flares with significantly faster reload time for balance, while helicopters completely lack the ability to carry and use flares. Flares in Assault Horizon should be used as a last resort measure to dodge missiles that are very difficult or impossible to evade with the normal way due to the very long reload time and very small number of flares available to the player, such as Markov's missiles in "Akula", or enemy's fully-charged missiles in DFM. However, flares are not foolproof. They should be deployed at the last moments before the enemy missile makes impact with the player's aircraft for maximum effectiveness. Flares that are deployed too early will fail to spoof the enemy missile. In the online multiplayer modes of Assault Horizon, due to the heavy usage of DFM, players are given more flares with faster reload time comparable to that of bombers' flares. When flares are deployed, all types of planes release a salvo of three flares, while bombers launch a small salvo of "Angel Flares". For balance purposes, the flares can be used to spoof not only the infrared guided normal missiles and QAAM, but also the radar guided missiles such as SAAM, 4AAM, and 6AAM.
In Assault Horizon, flares are also used by enemy planes. All enemy planes have infinite amount of flares with zero reload time, which means they can launch a salvo of flares right after launching one. "TGT_LEAD" planes clearly show this characteristic. They will constantly deploy flares if they are attacked before they complete their scripted flights. There is a known bug in the mission "Aftermath" where the final Tu-160 will constantly release flares to break the missile lock, effectively making the bomber near-impossible to shoot down due to its erratic movements, Markov's existence in the battle, and very short amount of time limit given to shoot down the bomber.