Image copyright Getty Images Image caption Barack Obama has called for a wide-ranging approach to mass violence, but it is clear he expects a proposal for a ban on assault rifles

President Obama has, whether by design or not, chosen a fight on one of the issues that will define how we look back on his presidency.

The obscene massacre in Newtown shocked the nation. Yesterday, at lunchtime, I heard a man outside one of the shrines in the centre of town saying that he hadn't felt like this since 9/11.

This is not a pivotal moment on that scale, but it could be important nonetheless.

The president said he hoped memories were not so short that the shock of the killings had faded in a month's time when he expects a report on his Oval Office desk.

Preventing some of the worst consequences of America's love affair with guns would be a very big legacy indeed

Many are cynical about this, and doubt the president's rhetoric will amount to much. But he will be hung out to dry if it doesn't.

He has gone out of his way to say that he wants action, not just words, and expects a series of concrete proposals that he will put in his State of the Union address at the end of January.

The president has called for a look into mental healthcare, video games and the culture of violence. But it is clear he expects Joe Biden to recommend a ban on assault rifles.

That means he is taking on a huge political battle that will rage throughout the year and probably for many years to come.

I am intrigued, more generally, by Obama's political game. Taking on Republicans in Congress, and then pointing out they stopped him from doing much of anything, doesn't have a lot of traction.

It worked, just, before the election. But in a second term, unless he expects to win back the house in the mid-terms, it won't earn him a place in the history books.

But preventing some of the worst consequences of America's love affair with guns would be a very big legacy indeed.