From what I've read the biggest threat that feared was saboteurs and the fleet didn't expect a large scale threat from the air. It just seems like AA weapons would be standard operating procedure for all of the airfields in the area. Was the Army just not concerned with threats from the air so they didn't have adequate AA guns? Was there something else that prevented the army from putting up stiff AA resistance?
The Army began dispatching the 251st Anti-Aircraft Artillery Regiment to Hawaii in 1940, but by the time of Pearl Harbor the 251st's strength was effectively at less than half-strength: less than 100 fixed and mobile 3-inch guns and about 130 smaller machine guns.
The Army had no planes in the air on December 7. Ammunition for the 37mm guns had only arrived two days before, and there was very little machine gun ammunition. In fact, a number of the 3-inch guns were stored on private property and the troops were only allowed to get close to them during occasional practice sessions.
The Army had three fixed radar sets in the process of assembly, and six mobile sets which were only turned on for a few hours a day. Only one mobile set picked up the Japanese fighters, and its crew didn't manage to warn HQ in time - because they were limited to commercial telephones.
On December 7, most of the military personnel in Hawaii were sleeping or lazily beginning a Sunday morning. No one was manning the AA defenses at Wheeler Field. Security there was light - but due to paranoia over Japanese saboteurs, the fighters were parked in a tight cluster unarmed so they'd be easier to guard.
Despite all of this - troops stood down, no warning, short ammo, no training - several AA positions were firing on the attacking planes within a few minutes. A great number of the Japanese planes shot down, lightly armored and flying low, were taken out by American soldiers, sailors and Marines firing rifles and pistols.
The Japanese lost nine planes in the first wave, and 20 in the second wave. This is a large part of why Admiral Nagumo didn't press his advantage and return for a third wave; he feared that a fully active U.S. defense would inflict terrible losses on his remaining flight crews (especially since he didn't know where the American carriers were). Throughout the war, Japan never managed to develop a deep bench for its carrier pilots; experienced pilots were kept in combat until they died. As Nagumo well knew, Japan's ability to project power and protect their carriers depended on a very thin line of pilots. Indeed, the loss of Japanese veterans at Midway and the Battle of the Coral Sea severely damaged the effectiveness of Japan's remaining aircraft carriers.
So: The American military (both Army and Navy) were taken by surprise but recovered quickly enough to force the Japanese away, thereby preventing the loss of oil tanks and other infrastructure that could have prolonged the Pacific war for months or years. Why were they surprised?
The Navy's commanders were smugly convinced that the Japanese could not mount an attack on Pearl Harbor, given its distance from Japanese waters. They believed (despite mounting evidence from its own wargames and British reports from Europe) that the waters were too shallow to allow an effective torpedo attack. Further, the United States had important bases closer to Japan which the Navy expected would be reduced before any attack on Pearl Harbor; the large U.S. bomber force in the Philippines, which threatened Japan's naval lifeline to its massive army in southern China, and Wake Island, which was in the process of being heavily fortified.
In the event, Japan managed to paralyze Wake Island's defenses with a heavy bombing attack shortly after Pearl Harbor, and another Japanese task force destroyed the Philippine bomber force on the ground.