Why heavy reinforcement still has to cross by sea

Personnel can cross the Atlantic quickly by air, but a modern army also needs ammunition, fuel, missiles, guns, tanks, and armoured vehicles. Air freight is too expensive and too limited in volume to sustain divisions, while airfields and transport aircraft face attack and aircraft require substantial maintenance between flights. Ships also need maintenance, but much of it can be performed underway, and a few ships can deliver a division's equipment. The crossing takes days or weeks, so that capacity comes with a long and vulnerable supply line. Anti-submarine defence of the GIUK Gap therefore protects the supplies that turn arriving personnel into an effective fighting force.