In the White House, they appeared almost stunned watching TV reports from Egypt. President Barack Obama aboard his jet Air Force One was kept up to date through a telephone briefing from the head of the National Security Council.

On the ground he told students: "What is absolutely clear is that we are witnessing history unfold."

That's is a pretty safe prediction.

He added: "It is a moment of transformation that is taking place because the people of Egypt are calling for change."

Again, little doubt about that.

The Egyptian army almost certainly felt events were slipping away from its control, and that it has to act because of the demonstrations and protests.

But the next part of his brief remarks commits his administration: "Going forward, we want those young people and we want all Egyptians to know that America will continue to do everything that we can to support an orderly and genuine transition to democracy in Egypt."

That means the president's team will be keeping a wary eye on their friends on the Egyptian army.

If Hosni Mubarak goes, the White House will see this as a beginning, not the end.

Western diplomats believe the Egyptian army saw the demonstrations growing and the demands from America hardening and feared that events were running away from them.

If Mr Mubarak goes it will be seen here as more than a simple symbol that the demonstrators' demands are being met. It will be seen as a vital removal of an obstacle that was getting in the way of the opposition engaging in talks.

And that is important to America. Perhaps the most important thing for the administration is stability and continuity, not least because Egypt is a friend of Israel and supporter of the peace process.

But that doesn't mean they will be happy if the army merely takes over and people leave the square. They are stressing the importance of real change and free elections.

This is not because of an abstract commitment to democracy, which many observe has been lacking in the past. It is down to a worry that if the army tries to cling to the status quo, there will be more demonstrations further down the line, perhaps with an added anti-Western tinge.

America craves stability in Egypt. But Mr Obama believes the only way to achieve it is through change.