What were the deciding factors of Pearl Harbor?

Tactically, the deciding factors of Pearl Harbor were that the Japanese achieved surprise and the Americans were completely unready for war.  Strategically, the deciding factor of Pearl Harbor was that the American carriers were not in port.

Tactics have to do with who wins the battle at a given time and place.  In other words, these are the factors that allowed the Japanese to win the battle at Pearl Harbor so decisively.  The first factor was that the Japanese achieved complete surprise.  The US knew that Japan was going to attack somewhere, sometime, but it did not know anything beyond that.  The Japanese fleet approached from an unexpected direction so American search planes did not detect it.  Additionally, it had maintained radio silence well enough that the Americans had no idea that it was coming.  This meant that the US was not expecting the attack in any way.

In general, the US was also woefully unprepared for war.  The US forces in Hawaii were not on a high state of alert.  This made it easy for the Japanese to overwhelm them after the attack began.  For example, US fighters were famously lined up in the middle of runways to prevent sabotage, making them easy targets for the attacking Japanese.  In addition, because the US did not expect an attack, they did not even take it seriously when radar detected the Japanese airplanes.  This lack of readiness, combined with the Japanese surprise, decided who would win the battle on December 7, 1941.

Strategy has to do with the bigger picture.  Strategy often determines who wins wars as opposed to battles.  Strategically, the US didn’t really lose at Pearl Harbor because its aircraft carriers were not in port.  Because the carriers were not there, they were not so much as damaged in the attacks and were free to fight the Japanese.  As it turned out, aircraft carriers were the decisive naval weapon in the Pacific.  If the Japanese had destroyed the American carriers, the war might have turned out very differently.  As it was, the carriers survived and went on to crush the Japanese at Midway about six months later, turning the course of the war.  Therefore, the deciding factor of Pearl Harbor in strategic terms—the factor that decided what Pearl Harbor meant for the war as a whole—was the absence of the US carriers.