have just led

Put yourself in the shoes of the emperor. You’ve just led your country through a disastrous war. The economy is shattered. Eighty percent of your cities have been bombed and burned. The Army has been pummeled in a string of defeats. The Navy has been decimated and confined to port. Starvation is looming. The war, in short, has been a catastrophe and, worst of all, you’ve been lying to your people about how bad the situation really is. They will be shocked by news of surrender. So which would you rather do? Admit that you failed badly? Issue a statement that says that you miscalculated spectacularly, made repeated mistakes, and did enormous damage to the nation? Or would you rather blame the loss on an amazing scientific breakthrough that no one could have predicted? At a single stroke, blaming the loss of the war on the atomic bomb swept all the mistakes and misjudgments of the war under the rug. The Bomb was the perfect excuse for having lost the war. No need to apportion blame; no court of enquiry need be held. Japan’s leaders were able to claim they had done their best. So, at the most general level the Bomb served to deflect blame from Japan’s leaders.

Could you please let me know why the perfect is used instead of the past, and what 'just' means?