DAYS AFTER a gunman carried out a horrifying attack on two mosques in Christchurch, New Zealand, that killed 50 people, Prime Minister Jacinda Ardern announced that she had ordered an inquiry to determine whether government agencies could have prevented the tragedy. “The purpose of this inquiry,” she said, “is to look at what all relevant agencies knew — or could or should have known — about the individual and his activities, including his access to weapons.”

How did this happen? What could have been done differently? The questions are simple and obvious. They are nonetheless still striking for the contrast they show between how New Zealand and the United States respond to mass shootings. Instead of asking questions or resolving to take action as politicians in New Zealand are doing, we have come to accept as inevitable that large numbers of people will be killed every year by madmen with guns. “Routine” was the lament used by President Barack Obama as he addressed the country in 2015 after nine people were murdered at a community college in Oregon; it was his 15th address after a mass shooting.

Such mass shootings are rare in New Zealand, as are all gun-related homicides. But the ease with which the Christchurch gunman was able to obtain an arsenal that included semiautomatic weapons was enough to convince Ms. Ardern of the need for better checks on guns. She announced her intent to tighten the country’s laws mere hours after Friday’s terrifying events. She announced Thursday — six days after the attack — that the country will ban semiautomatic weapons, assault rifles and high-capacity magazines. She said legislation is being drafted with urgency and hopes for implementation by April 11.

“The time to act is now,” she said this week.

Parliament will have to give its approval and there may be opposition, but the gun rights lobby in New Zealand is far less powerful than that in the United States, where the National Rifle Association’s stranglehold on lawmakers blocked any meaningful reform even after elementary school children were slaughtered. And it was encouraging — even inspiring — to see politicians in New Zealand who had opposed gun control change their minds. “The reality is that after 1 p.m. on the 15th of March, our world changed forever, and so will some of our laws,” said Deputy Prime Minister Winston Peters, whose New Zealand First party has previously opposed changes.

Meanwhile, in Washington, the modest push for universal background checks for gun purchases that passed the House faces an uphill struggle in the Senate and veto threats from the White House, even though it has the support of most Americans. Once again, the U.S. political system looks feckless compared with other democracies.