REWARDS - Respect
If you don't have a military vehicle, don't worry. Head to the yellow blip on the map. Wait for the gates to open up to let the Patriot out, then head inside. The guys in the base are packing some serious heat, so bring an assault rifle and don't forget armor. Take out any guards around the gate, then make your way to the warehouse on the left side.
Carefully make your way to the opposite entrance, taking out guards on the ground and on the catwalks. Once you make sure that everyone is taken care of, head outside. If you need health/armor, there is one pickup of each on the ground floor in the open. Once you're outside, carefully make your way to the steps located to your left. Continue to make your way to the roof, where you'll eventually see the chopper you need. You'll get a message that two gunships are on their way to intercept you, so step into the red marker to man the gun emplacement.
Once you destroy both of the choppers, climb the steps and get into the chopper. Move towards the blue blip on the radar to pick up the van. Hover over the van and position yourself to grab it with the magnet. Once you have it, head to your airstrip. Be careful with the van; try not to hit anything with it. There's no need to rush with the van, so take your time to get to the airstrip. Once you get there, carefully put the van in the red marker and then land the Leviathan in the indicated area to finish the mission. Respect earned.