One of the two candidates will soon be sworn in as President of the United States, to "faithfully execute" that office to the best of his ability, and to "preserve, protect and defend the Constitution of the United States." When the fervor of the elections dies down and the bumper stickers are forgotten until the first disappointment, America and its new leader will enter one of the most complex periods in its history.

In the presidential debate about a week ago, John McCain and Barack Obama were asked what things they do not know, and how they intend to learn them. Obama said the challenges the American people face are enormous, and one of the things that is known about the presidency is that these challenges are never the ones you expect. McCain more or less reiterated that idea, saying that he does not know the same thing everyone else does not know: what is going to happen at home and abroad.

The 44th president will obviously not be the first to take the reins of government in volatile times. Lincoln jumped straight into the Civil War; on Franklin Roosevelt's desk were not flowers, but rather the dreaded depression; Truman asked people shortly after he was sworn in, ahead of the end of World War II, to pray for him; Kennedy inherited the Cold War and Eisenhower had the Korean War; Nixon, who solemnly promised during his presidential campaign to end the war in Vietnam, from the moment he was elected had to figure out how, in fact, to go about that; and Clinton was asked to fix the economy and decide what to do with the bloody trap of the Balkans.

The next president will get something of everything: two wars, a sky-rocketing national debt, a limping economy, an obdurate Iran, a manipulative North Korea, an ambitious Russia and China, and many worried Americans.

In addition to the known problems, reality likes to surprise a new president: Suffice to recall that the current president pledged to be as little involved as possible in the affairs of other countries - until September 11. Or, as McCain now puts it, there are new challenges around the world and in the future we will talk about countries that we hardly know exist.

The period of transition while the administration changes is a powerful shock with many dangers: The warning that hostile elements might take advantage of these 72 days to promote their interests must be taken into consideration, and a new president trying to differentiate himself from his predecessor might make a hasty move. For months, the two candidates have been quietly examining potential members of their administration. But at the moment of truth, the appointment process rarely flows smoothly. In this super-casting, the deliberation is between loyal staff members and advisors, and stars from the outside; between veterans who know the material and the need to initiate - or at least proclaim - the change to which both candidates have committed themselves.

After eight years of Bush government, putting together the huge administration puzzle will affect the entire system. Think tanks can suddenly become revolving doors - with senior people leaving government having to lecture and write books, while those who are called to serve will leave their posts to other ambitious people. Administrative assistants will have to look for other jobs, no less worried than the senior officials they worked for. Will the charming press corps that coalesced in the days of Condoleezza Rice to cover the State Department (known as the "diplo-babes") survive if a man fills her shoes?

The main mystery on McCain's team is Sarah Palin, the big temptation is to appoint old friends, and the biggest dangers are ego-wars and over-daring moves, like those the candidate made during the campaign. Obama's team is more consolidated and disciplined than McCain's, but there are too many people there who will be glad to have a suitable post as a reward, and it will not be easy to decide. It may be assumed that the biggest change will be less one of essence and more of image. In terms of economics, the big corporations have a reason to give generously to the democratic candidate: He can help get rid of the "damaged Republican brand," but it is doubtful he will be able to shake up the system.

Pacifists are also likely to be disappointed: Obama opposes the war in Iraq mainly because it is the "wrong war at the wrong time;" he has already pledged not to hesitate to use force anywhere American interests are under attack or real threat. Some of his close associates are among the supporters of "humanitarian intervention," and Darfur could obviously have been a candidate for the next intervention if the army were not already stretched to its maximum, possessing the most advanced weaponry but with fewer available soldiers than commanders in Afghanistan would want.

Will Obama talk to Iran? The idea has been cooking over a low flame in the Bush administration, and the recent approval given to an American organization to open a branch in Tehran is another step in that direction. Obama, if he wins, will first of all change the whole style of diplomacy, and will command a great deal of trust. If the crowds who gathered to see him on his international trip last summer had the right to vote, he would have been president for two months already. No doubt the change in diplomatic style could have a significant positive effect on the status of the United States in the world. In its present circumstances, that would mean a lot.