General David Petraeus’s resignation as director of the US Central Intelligence Agency is a heavy blow to President Barack Obama. As he prepares for his second term in the White House, Mr Obama knows he will confront a range of tough national security challenges, such as the stand-off with Iran, the Syrian civil war and US withdrawal from Afghanistan.

On all of these, Gen Petraeus, widely respected for his service in Iraq and Afghanistan, would have been an important source of advice. His resignation over an extramarital affair – at a time when other leading figures are already set to leave the administration – is something Mr Obama could ill-afford.

For now, much attention is on the circumstances surrounding the general’s affair. There are still questions to clarify, such as why the FBI failed to notify the White House and Congress immediately after it discovered details of the relationship. But as he prepares to choose a new CIA director, the president might also take the chance to review the agency’s counter-terrorism strategy, reining in its excesses.