"If there's another 9/11 or a major war in the Middle-East involving a U.S. attack on Iran, I have no doubt that there will be, the day after or within days an equivalent of a Reichstag fire decree that will involve massive detentions in this country"

This quote by Daniel Ellsberg is referring to the potential effects of a significant occasion such as a 9/11 or a war in the Middle-East including a U.S. attack on Iran. Ellsberg is cautioning that if such an event were to take place, the U.S. government would likely respond with a "Reichstag fire decree" which would include the mass detention of individuals in the United States. This is a referral to the Reichstag Fire Decree of 1933, which was released by Adolf Hitler in reaction to the burning of the Reichstag structure in Germany. The decree suspended civil liberties and enabled the mass arrest and detention of political opponents. Ellsberg is cautioning that a comparable decree might be provided in the United States in reaction to a major occasion. He is cautioning that such a decree might result in the mass detention of people in the United States, which it is essential to be familiar with the prospective repercussions of such an occasion.