At Thursday’s Democratic presidential debate in Milwaukee, former Secretary of State Henry Kissinger became a surprise subject of contention.

Sen. Bernie Sanders criticized former Secretary of State Hillary Clinton over her friendly ties to President Nixon’s foreign-policy guru. Clinton wrote a book review in October of Kissinger’s “World Orders,” calling him a friend and saying that she consulted with Kissinger while serving in the White House. Clinton also made a passing reference to Kissinger in a previous debate, saying she was “flattered” when he said Clinton ran the State Department better than anyone had in a long time.