Soon after the US presidential election Hillary Clinton was presented with a choice: to stay in the Senate as one of the most powerful women in America, or become Barack Obama's Secretary of State, and one of the most powerful people in the world. She chose the latter.

The cost to Mrs Clinton's substantial self-esteem will, paradoxically, be high. She will have to subordinate her world view to President-elect Obama's. She will have to rein in her husband, a global statesman in his own right. And she will have to shelve any presidential aspirations of her own that she may still harbour.

But the stakes - and potential rewards - for America's top diplomat over the next four years are even higher. She will lead the effort, so keenly awaited in so many countries, to restore what Mr Obama called yesterday “the power of our moral example”. Since 2003, a war of choice in Iraq and the debacles of Abu Ghraib and Guantánamo have left that power to atrophy. Rebuilding it would be critically important to the country's image and self-image at the best of times. At a time of crisis for free-market liberalism everywhere, it must rank as a central goal of Western civilisation.

Specific challenges will be thrust on Mrs Clinton from Day 1. Mr Obama has vowed to restart the Middle East peace process after the Israeli elections in February. He also plans an ambitious encirclement of al-Qaeda in Pakistan, and must forge a new relationship with Russia to have any hope of concerted international action against Iran's nuclear enrichment. His will be the face of change presented to the world, but the heavy lifting will be outsourced to Mrs Clinton. Her relationship with him will be her most important.

As a candidate, Mrs Clinton authorised a now-famous television advertisement that asked voters whom they would trust with a 3am “crisis call” to the White House. The ad backfired. When the call comes, Mr Obama will take it and Mrs Clinton - who has insisted on direct access to the President for fear of being sidelined by his National Security Adviser - may be the one making it. During the campaign she also called Mr Obama naive for contemplating talks with Iran. To be effective, she will have to swallow her words - and some of her pride.

Henry Kissinger, George Schultz and James Baker, the most successful recent secretaries of state, derived their power from their closeness to their presidents. Mrs Clinton may not adopt the same role of consigliere to hers, but if she is to “command respect in every capital”, as he predicted yesterday, they will have to speak with one voice on every major issue.

Differences between the two on Iraq and Afghanistan have narrowed predictably since the election. Taming the former President Clinton may prove more problematic. He has paid his own price for his wife's promotion, agreeing to close scrutiny of his speeches and charity work. He may regard informal influence over US foreign policy as his quid pro quo. He has not earned it. Furthermore, his is the face of the 1990s, not of change, and he retains advisers whose experience was gained when America's moral stature still went unchallenged.

Times have changed. Mrs Clinton must keep her husband at arm's length, and learn to see the world through her new President's eyes.