By The People - The Election of Barack Obama, BBC Two / Simon Schama on Obama's America, BBC Two

Saturday evening's By The People - The Election of Barack Obama helpfully illustrated some timeless truths about the art of documentary film-making. Its co-authors, Amy Rice and Alicia Sams, had spent two years enjoying priceless backstage access to Barack Obama's campaign for the presidency, first as he saw off Hillary Clinton for the Democratic nomination and then during the Presidential campaign itself.

Then they were allotted a priceless two-hour broadcast slot, which is tantamount to film-makers' nirvana. Yet, loaded down with all the goodies the documentary gods could provide, they went and made a film so dull and preachy that I could sense my television blushing with shame for having to show it.

What a pity, though I couldn't help sniggering at the thoroughness of their self-deception. At the time they must have felt, like our own daydream believer Tony Blair, that the hand of history was on their shoulders as they recorded Obama's triumphant march on Washington. In fact if I'd watched this film a year ago, I might have been swept up - well, a bit anyway - in its aura of airbrushed euphoria. Instead, the passage of time, and the way all the little bulbs in Obama's halo have started to splutter and burn out, made By The People look like a crass hagiography made by a PR agency, waist-deep in treacly self-righteousness and apparently truly convinced that a solitary politician, albeit an exceptional one, was going to strip and rebuild the unfathomable innards of the American political machine overnight.

It was like being enveloped in a creepy conspiracy of mushy liberal utopianism

I'm still prepared to believe that Obama does have some genuine ideals and a real vision for his country, but the longer this went on the more I could feel certainty slipping away. The bit where he's out campaigning and phones home to talk to Michelle and the kids? Well, what a bit of luck the cameras and sound equipment were all set up at his house. Then his grandmother dies, and he promptly delivers a weepy speech about America's "quiet heroes" and how "we are one day away from changing America". It was like Forrest Gump. And when Bruce Springsteen sang "This Land Is Your Land" over the end credits, there was that desperate sensation that the bodysnatchers had got to the Boss as well. It was like being enveloped in a creepy conspiracy of mushy liberal utopianism, where the only dissenting voices belonged to gun-toting extremists who cackled manically as they tore up Main Street in their gas-guzzling SUVs.

If nothing else, it went to show that the best documentaries are never the ones where the story is handed to you on a plate with a dollop of whipped cream, but where you have to fight tooth and nail for every minute of footage. Even when you can't get all the stuff you want, you battle to make a film with some attitude and a bit of backbone.

After this I didn't expect much from last night's Simon Schama on Obama's America. Schama's pushy omniscience and his affiliation to the Robert Peston school of vaingloriously contorted vowels can be enough to push you over the edge after only 10 minutes. But it turned out to be a thought-provoking essay on the way America has reached a fork in the road with the war in Afghanistan. What price should the USA pay for its freedom, Schama (pictured below) wondered. Could it afford to withdraw, let the Taliban return and see Afghanistan revert to being an Al-Qaeda playground, or should it commit the resources and manpower to force lasting change?

Schama reckons Obama is a knowledgeable student of America's history, and aware of the lessons from previous wars. He took us back to the slaughter the Americans suffered on Omaha Beach on D-Day and how it was viewed as a worthwhile sacrifice in crushing the Nazi tyranny. The good-quashes-evil nature of World War Two, Schama added, has tended to convince Americans that all their warlike interventions can be similarly clear-cut and righteous (one observer described it as the "Saving Private Ryan complex"). But then he waved the hideously murky and compromised lessons of Korea and Vietnam in our faces. Harry Truman backed down from all-out war with China in the former to prevent global apocalypse, while Vietnam became a generic term for carnage and futility.

Zbig Brzezinski, formerly Jimmy Carter's National Security Adviser, warned bleakly that an endless war of attrition in Afghanistan could mark the beginning of the end of America's global influence. But Schama pointed to the example of South Korea, which has boomed under American protection while its northern neighbour has descended into nuclear-armed psychosis.

Obama, derided by his critics for his naivety and lack of political experience, came to power in a rush of youthful idealism and hope. Now, posits Schama, it falls to him to make the decisions that may determine whether America as we've known it lives or dies.