The men who wrote the American constitution included its goals right there in the preamble: Establish justice. Ensure tranquility.  Secure the blessings of liberty. Lately, there have been plenty of reasons to question whether that document is living up to those goals — and there's real talk of a constitutional crisis. At a moment like this, when the future looks uncertain, Harvard historian and law professor Jill Lepore is an expert at explaining why history matters. We talk to her about her latest book, "We the People: A History of the U.S. Constitution."