"Now that the cold war is over, Israel has become a strategic liability for the United States," they write. "Yet no aspiring politician is going to say so in public or even raise the possibility" because the pro-Israel lobby is so powerful. They credit the lobby with shutting down talks with Syria and with moderates in Iran, preventing the United States from condemning Israel’s 2006 war in Lebanon and with not pushing the Israelis hard enough to come to an agreement with the Palestinians. They also discuss Christian Zionists and the issue of dual loyalty. (Patricia Cohen, "Backlash Over Book on Policy for Israel," New York Times 16 August 2007)The question that Mearsheimer and Walt have raised is a valid question, and their raising of it, given their institutional stature in academia, has helped open up a political space for necessary debate on the US-Israel relation in a way that no writer on the Left could.
That said, whether Israel has become "a strategic liability for the United States" can be only answered by asking another question first of all: what are the goal and strategy of the power elite of the United States? Depending on the answer to that question, Israel may or may not be a liability.
Moreover, "now that the cold war is over," has anything fundamental changed in the US power elite's outlook? The USA is still as much of an empire as it was during the cold war, and its policy toward not just countries with considerable assets such as Iran and Venezuela but very poor ones such as Cuba, Nepal, and Haiti is still the same: make sure the power elites of countries in the South are pro-Washington, by any means possible. If the US policy elsewhere hasn't changed, why should it regarding Israel/Palestine?