Nothing could be further from the truth. Defense is actually the #1 Constitutional duty of the federal government, a duty which the said government is obliged to carry out to the best of its ability. Furthermore, the Founding Fathers were nearly unanimous (a few skeptics like Elbridge Gerry being exceptions to the rule) about the need for a strong defense, as expressed by George Washington, John Adams, James Madison, and many others.
But in general, those who call themselves libertarians (Paul, Rothbard, ect) or Paleoconservatives (Buchanan) are really little more than anarchists or just populists in the case of Buchanan.
Rothbard is a very curious figure. He is off the reserve when it comes to the Republic and self defense and acting like a carpetbagger for our enemies. But he was a great economic thinker.
Buchanan is hardly even conservative except on social issues. The man is obviously motivated by his Irish heritage in his hate for a great man like Churchill and the way he constantly trashes our English allies in favor of the Germans.
Others like Paul are just deluded. They have somehow stretched the constitution to equate it with hating their own country. As much as they decry those on the left and right who ignore the Constitution, the Pauls of the world abuse the constitution for their positions. Jefferson was small government, but he did not hate his own country. He was generally against foreign alliances but he did see fit to bring war to the Moslems in Africa.