A QRF, or Quick Reaction Force is triggered when Capturing towns or Capturing Bases. NATO will use the available vectors to send forces by land, sea and air. A 10 min timer will start, after which points will be calculated according to the number of forces from each side within 100m of the battle marker. Players count as 2 points and all others count as 1. The total points required to win depends on the size of the force sent by NATO. A circle will appear on the map and show who is currently winning the battle, Blue = NATO, Green = Resistance. The circle will become darker as points are gained for each side.
Reducing QRF size
NATO has an available amount of resources that are used for QRFs as well as missions. You can keep these resources down by going on the offensive often and killing any HVT (high value targets) officers that you see stationed at Bases.
NATO will only use the vectors available to send troops. For example, if you are attacking a town on an island with a Base, you can expect ground forces and support to come from that base and plan your defense accordingly along roads leading from that base. If there are no bases on the island or they are controlled by the resistance then you will only get forces sent by helicopter. If the battle is occurring near the ocean then you may also get gunboats from the closest NATO navy base.
It therefore pays to prioritize capturing all the bases on an island to reduce QRF size.
Defending bases and towns
If NATO has resources to spend they may decide to try and re-capture a town or base controlled by the resistance, triggering a QRF for that location.