American diplomats should be proud of the new sanctions on North Korea that the United Nations Security Council passed last week. In response to a nuclear test in January and missile launch in February, the Council, at the United States’ urging, agreed to a ban on North Korean imports of jet fuel and to mandate inspections of all cargo coming into or out of North Korea. These are the toughest sanctions the North has ever faced.

But before celebrating the beginning of the end of North Korea’s nuclear and missile programs, it is worth remembering the recent history: Since 2006, the Council has passed five major sanctions resolutions. In 2013, the United Nations adopted sanctions that were, at the time, the toughest ever imposed on North Korea. And yet the nuclear tests and missile launches continue.

What accounts for this? Some people argue that North Korea’s leaders are not just intransigent but irrational. (We believe that is an inaccurate characterization.) Others say the problem is that individual governments are left responsible for enforcing sanctions. (They have a point: A United Nations panel found last month that many governments have failed to implement sanctions that are already on the books.) And some policy makers blame China for blocking even more stringent sanctions that could bring North Korea to heel. (It’s true that the government in Beijing fears that too much sanctioning could cause a North Korean collapse, leaving a nuclear-armed failed state on China’s border.)

But there’s another reason that North Korea, in spite of years of sanctions, has proceeded with its weapons programs: The government has found sophisticated and effective ways to evade sanctions thanks to what we call North Korea, Inc., a cadre of elite state-owned trading companies that keep the flow of cash and supplies steady.