THE second amendment to the United States' Constitution, adopted in 1791, is not quite the elegant prose of the preamble (''We, the people …'') or the first amendment, which protects free speech and assembly. It is a clunky and disjointed run of phrases that invites alternative interpretations. The confusion at the heart of the amendment - whether the right to ''keep and bear'' firearms depended on the individual being part of a ''well-regulated militia'' - was debated in the US Supreme Court in 2008, for the first time in almost 70 years. The case emerged after an appeals court in Washington DC struck down laws that banned people from having handguns in their home (as opposed to rifles and shotguns which, curiously, were permitted). The Supreme Court agreed with the appeals court. It found the ban affected a particular class of firearms which Americans ''overwhelmingly choose for the lawful purpose of self-defence'' and extending that ban ''in the place where the importance of the lawful defence of self, family, and property is most acute'' was unconstitutional.

Once again, the court perpetuates the standing fallacy in the US that firearms are indispensable because they provide protection against danger. It is a specious concept, one that fosters the extraordinary proliferation of guns in America and which pays no mind to the fact that most people in the rest of the world seem to do rather well without them. How is it, for example, that 74 per cent of homicides in North, South and Central America and the Caribbean are caused by firearms compared with only 21 per cent in greater Europe? Dispensing with the casuistry that posits firearms as defensive tools, rather than offensive, is the key to changing the violent gun culture in the United States, but it is not something that can be done in a single generation.

President Barack Obama is taking small steps. One week after 20 young children and six teachers were killed in a bloody rampage at a Connecticut school, Mr Obama has vowed to put new gun control measures to Congress ''no later than January''. What that entails is not yet clear. It may end up as merely a ban on assault weapons, such as rapid-loading semi-automatics. It may be more. He has commissioned a task force, led by Vice-President Joe Biden, to examine the options and he has promised its recommendations will not fall by the wayside.

It is imperative that Mr Obama seizes this moment and forces dramatic change. It is what prime minister John Howard did in 1996 when, in the wake of the Port Arthur massacre and just two months after he had been elected, he banned assault rifles, imposed mandatory licensing and registration of firearms, and declared ''We do not want the American disease imported into Australia''. The Age salutes Mr Howard for that brave move.

Now, more than ever, Mr Obama has the moral authority to do at least a portion of this, an authority that is vested by virtue of yet another dreadful incident. Any less would mean failing the schoolchildren of Connecticut and their families, as well as the children of future generations. Already the US has endured too much mindless carnage. From 1999 to 2010, the second leading cause of death among 15 to 24-year-olds in the United States was homicide. Four in five of those homicides (51,384) involved a firearm. A further 3000 American children were shot dead in that time before they turned 15.

If Mr Obama is half the president he promised he would be, then he must galvanise forces across the political spectrum and make this the signal change of his administration. He must make ''yes, we can'', which still rings, into ''yes, we did''. This is the threshold moment.