Was it to protect national security?
That would be a "NO".
It was to save his puny ass:
...Bush was desperate to keep the Times from running this important story—which the paper had already inexplicably held for a year—because he knew that it would reveal him as a law-breaker. He insists he had “legal authority derived from the Constitution and congressional resolution authorizing force.” But the Constitution explicitly requires the president to obey the law. And the post 9/11 congressional resolution authorizing “all necessary force” in fighting terrorism was made in clear reference to military intervention. It did not scrap the Constitution and allow the president to do whatever he pleased in any area in the name of fighting terrorism.
Attywood already discussed this as well, but the sound and fury from Bush and his pathetic right-wing enablers is so loud -- as always -- that it obscures the frightening truth, Bush is more than just a mere empty suit, he is a loathsome scofflaw, abusing the constitution and the long-held sacred institutions of a Constitutional Democracy.
The Republic falls in stuffed flight-suits and stuffed dolls.