On Oct. 2, Saudi journalist and U.S. resident Jamal Khashoggi — a critic of his native country’s regime — walked into the Saudi consulate in Istanbul to file some paperwork. No one has seen him since.

To all appearances, and absent any explanation to the contrary, the Saudi regime is responsible. Its intelligence service sent an entire team into Turkey to conduct the operation. They may have only intended to kidnap Khashoggi, but he is now feared dead and perhaps dismembered as well.

This would mean that Saudi Arabia, never an exemplary country when it comes to human rights, has taken its perfidy to a whole new level. It has followed in the footsteps of Russia and China in flagrantly murdering and kidnapping people on foreign soil, in contravention of the rules-based order that world powers have tried to foster.

In a dangerous world and an especially dangerous region of it, the U.S. cannot afford to lose Saudi Arabia as an ally. Iran, which conducts far more deadly operations outside its borders on a routine basis, would love to see a breakup. And Russia’s Vladimir Putin is waiting with open arms in case this eight-decade relationship is severed.

In global politics, we can't steer away from the ugly and we can't demand all of our allies live up to the highest standards of human rights and democracy. Our alliances during the Cold War and World War II are perfect lessons in the ugly side of realist foreign policy.

[More: Trump can easily show true leadership on assassinated journalist Jamal Khashoggi]

But that doesn’t mean Americans can lightly accept, let alone defend, that their ally should engage in such vile crimes as this one. President Trump must not allow his sovereignty-oriented "America First" foreign policy turn into a blank check for further Saudi abuses. Not only will it stain the nation’s reputation, but it will also — perhaps more importantly in the short run — damage his own credibility if he lets this kidnapping and apparent murder pass with nothing more than a shrug.

The Saudis know that their alliance against a menacing Iran is of great importance to Washington. On that basis, they miscalculated that they could get away with this. They must now be made to understand just how badly they miscalculated and that the consequences are beyond what their regime can handle.

Trump must react first of all by applying pressure behind the scenes. Symbolic expressions of disapproval (limiting diplomatic interactions) must be accompanied by realistic threats that the U.S. will escalate if anything further happens to the regime’s critics. As valuable as Saudi intelligence and military cooperation may be, the U.S. must make clear that we are getting close to the point at which it will no longer be justifiable.

Once Trump is certain that Khashoggi was murdered, a public rebuke must follow. So must sanctions, along the lines the Senate envisions — not the ineffective kind that are laid upon the entire country, but the kind that specifically ruin and disrupt the lives of those known to be responsible.

An approach like this one has been the most effective in getting under the skin of Putin and his allies. It also keeps several bullets in the chamber, to be used as leverage against further bad behavior.

Trump must give Saudi Crown Prince Mohammad bin Salman a clear understanding that things can get much, much worse for him if he persists in this course of action.