The Portuguese Air Force shows us their Quick Reaction Alert (QRA) training at Monte Real Air Base and explains the state of readiness they have to maintain to be airborne within 15 minutes.

Quick Reaction Alert, known colloquially as QRA, is state of readiness and modus operandi of air defence maintained at all hours of the day by NATO. Fighter jets can be airborne within minutes to intercept and identify unknown aircraft operating within NATO airspace. Member countries provide the necessary aircraft and assets for the air policing of their own countries. For those countries that do not have this capability within or near NATO airspace, other NATO members will assist on a rotational basis.