President Donald Trump and his top aides have given America two main stories about the separation of families at our southern border: It's an unfortunate byproduct of laws and court decisions that tie the administration's hand, and it's a necessary deterrent to illegal immigration. These are conflicting accounts. If the administration believes the first one, it should welcome legislation to keep families together. If the second reflects its thinking, it should reject such legislation for undermining a valuable tool against illegal immigration.

Officials have not gone out of their way to clarify the issues.

Take, for example, Homeland Security Secretary Kirstjen Nielsen's recent comment: "We do not have a policy of separating families at the border. Period." Based on previous remarks she has made about the policy, what she appears to mean is that the administration is not separating families on purpose but doing it because it's what happens when you prosecute parents for crossing our borders illegally.

It's a defense of the administration that ignores three salient points. First, the policy of prosecuting all illegal border crossers as criminals — what the administration is calling "zero tolerance" — was a choice of this administration. Previous administrations considered it and rejected it. Second, it was chosen even though one of its predictable effects was to take many more children from their parents. That's a major reason it was previously rejected.

Third, at least some high-ranking officials favor the separation of families as a way to discourage illegal immigration. The White House chief of staff, John Kelly, told CNN in March 2017 that he was considering separating families as a deterrent. Attorney General Jeff Sessions has done his part to amplify the deterrent message: "If you don't want your child separated, then don't bring them across the border illegally."

Deterrence is a valid purpose of the law, but it has moral limits. Sessions would not, I trust, endorse executing the children of illegal immigrants to deter others from crossing. He would see the perversity of a statement like "If you don't want your child executed, don't bring him across the border illegally." The deterrent he is defending, while less severe than that, also crosses the moral line. It places much of its burden on children. To the extent it works as a deterrent, it does so precisely by harming them. That's why even in the Trump administration, very few people are willing to defend it on these grounds.

Nielsen wants Congress to amend the law to allow illegal border crossing to be prosecuted as a crime while keeping families together — which, presumably, would mean letting families be detained together for as long as it takes to see their cases through, and providing funding both to build facilities in which they can be humanely detained and to speed up their cases. Her implicit view is that zero tolerance without family separation is better than zero tolerance with family separation, but zero tolerance with family separation is better than what we had in place before this year.

Her view is defensible only if a small and uncertain reduction in illegal immigration is worth tearing apart families. But it too clashes with the sound moral instincts of most Americans. Maybe that's why she doesn't state it openly.

An alternative policy would have three parts. The administration would stop enforcing its zero-tolerance policy on adults traveling with children until it got changes in the law that enabled the humane detention of families. It would advance those changes in the law as a stand-alone measure rather than using legislators' desire to keep children with their parents as leverage to win other immigration-related policy changes. And it would, separately, push for legislative changes that enabled other means of enforcing the immigration laws, such as punishing businesses whose new hires are illegal immigrants.

Really, though, these three steps boil down to one: Just don't split up families when it's not absolutely necessary. Among the advantages of this simple alternative is that the administration would find it easier to keep track of its story line.

Ramesh Ponnuru is a Bloomberg Opinion columnist. He is a senior editor at National Review, visiting fellow at the American Enterprise Institute and contributor to CBS News.