Mr President-Elect, that was the easy part. Yes, becoming the first black President by a landslide - winning even the former capital of the Confederate slaveocracy, Virginia - didn't seem like a stroll at the time. But now there is a pile of ticking time-bombs waiting in your in-tray and you have to defuse each one of them, fast. Welcome to the next four years of your life.
Some of the challenges that define the Obama years will be impossible to guess at today. Who foresaw the rise of Hitler in 1932, or 9/11 in 2000? But the Oval Office desk is also littered with grenades we can already see.
Time-bomb One: A collapsing economy. The US economy is freezing like it's 1929 - and you have to decide now which wing of the Democratic Party can put it right. To your right, the Robert Rubin wing will tell you to concentrate on cutting the deficit, bailing out only the biggest, and batten down for the storm. They have the backing of the super-rich and their institutions. To your left, the populist wing will tell you to spend big - on healthcare, renewables and infrastructure - in order to revive the economy. They want a new New Deal - and they have the backing of most ordinary Americans. In the first year of the Clinton administration, the wrong side won, and Rubin was unleashed to deregulate the banks. You need to make a better choice. Make yourself Franklin Roosevelt Mark II. Which leads us to...
Time-bomb Two: A collapsing climate. You have become President at a crucial moment in the planet's history. We are close to the climatic Point of No Return: a two degree rise in temperatures, which will trigger an unravelling of all natural processes. The last two Presidents killed Kyoto. You can save its successor, which has to be negotiated before 2012. But that means you need now to bring the US - the worst per capita emitter by far - into line. The economic crisis gives you the perfect opportunity. Stimulate the economy by launching the transfer to a low-carbon economy: paint your New Deal green. Big Oil will fight back hard and dirty - but every human being needs you to fight back.
Time-bomb Three: America's wars. Since some 70 percent of Iraqis want the US troops out now, Prime Minister Nouri al-Maliki will be happy to negotiate your departure. The bigger danger will be in Afghanistan, where you say "success is crucial." What does success mean in a country which has never been under the central control of Kabul and has repelled invaders from Alexander the Great to the British and Soviet Empires?
You have to begin by doing something the Bush administration was incapable of: acknowledge that America's power to reshape reality has limits. You cannot clear Talibanism out of Afghanistan by force. They are too large, the land is too impenetrable, and the harder you try with brute force, the more Afghans you drive towards them. The US commitment to destroying the country's opium poppy crop super-charges the hate. What country would ever accept foreign forces committed to trashing 60 percent of their economy?
Here's your path out. Buy the opium crop and use it to make painkillers, as the US does in Southern Turkey. Then you would be approaching Afghan peasants not with guns, but cash. Then you will have to do something ugly. You are going to have to negotiate with the Taliban. All Taliban are despicable women-enslaving thugs - but you can't (alas) eliminate them now. No: you need to have a more modest goal. Virtually all intelligence experts agree there is a division between most of the Taliban, who have a local Afghan agenda, and Al Qaeda, who have a global jihadi agenda. You need to break chunks of the Taliban away from al Qaeda, so the jihadis are left isolated - and beatable. The elected Afghan President Hamid Karzai and now even General Petraeus are begging to do this.
You will be accused of appeasement by the right. They'll say you are leaving space for al Qaeda training camps. But if you continue on their preferred path, you won't just be negotiating with parts of the Taliban - you'll be defeated by them as the Afghan population turns irrevocably against you. Then the space for training camps will be larger still - and jihadis will claim victory and fight all the harder everywhere.
And those, Mr President-Elect, are only the starter-issues. You just made history once. You awed the world and our eyes water with optimism. But if humanity is going to get through the swelling crises waiting for us, you are going to have to make history again and again in the next four years - and we, the worried, watching people of the world, will be here pressuring you. This - the longer, harder campaign - has only just begun.
POSTSCRIPT: If you want a sense of how this election is being seen across the world in this euphoric burst, one little anecdote for me says it all. I spent yesterday trawling the shops here in London for Stars and Stripes to decorate my apartment for my Presidential election party - and across the city they were all sold out. One shopkeeper in the East End told me: "For the past eight years we've done a big trade in American flags because people buy them to burn them. This is the first time I can remember people buying them because they actually want to wave them."
Johann Hari is a writer for the Independent newspaper. To read more of his articles, click here.