The intensifying situation in Ukraine has presented President Biden with perhaps his most difficult foreign-policy challenge yet. Ukraine — a non-NATO, pro-Western ally 31 years free from the grip of Moscow — is once again staring down the barrel of its familiar foe, with over 100,000 Russian troops amassed on its border.

It was once unthinkable that Russian president Vladimir Putin, for all his Soviet nostalgia, would make a grab for the Donbas region of Ukraine. That threat is now imminent, and it jeopardizes not only Ukraine but free Europe as we know it.

For the sake of Ukraine and the broader region, Putin must be stopped before he fires a shot. President Biden can take several steps to compel him to stand down.

First, President Biden should make clear that there is no scenario under which Ukraine will be overrun by Russia, period. Putin is already courting a bloodbath should he attack Ukrainian troops. President Biden should up the ante by warning him that an invasion would saddle him with an intolerably high Russian casualty count. This means leaving all options on the table and granting no concessions.

Second, President Biden should arm Ukraine with the advanced weaponry it desperately needs. Russia is armed with droves of ballistic missiles and enjoys both sea and air superiority. This asymmetry means Russia could cripple Ukrainian lines before Russian troops even begin to pour in. The U.S. should offer anti-aircraft systems to protect Ukrainian skies. We should also continue providing Javelin anti-tank systems to deter Russian tanks from crossing the border.

Third, NATO should send additional troops to supplement Ukraine’s ground forces. Top Ukrainian military brass have made clear that, although they are willing to fight to the bitter end, it will take Western assistance to repel an invasion. A surge of NATO troops would deter Putin from invading while demonstrating NATO’s enduring value as a pillar of European security.

Fourth, to show he is serious, President Biden should sanction Russian officials associated with the Nord Stream 2 pipeline. This pipeline is a direct attack on the NATO alliance and is incompatible with a free Europe untethered from the will of Moscow. Far from admitting weakness, a reversal on this issue by President Biden would show strength in the face of a deteriorating situation.

Finally, the Biden administration should ratchet up sanctions on Putin and his inner circle. There is now growing, bipartisan appetite in Congress to tighten the financial screws on the Kremlin. Serious sanctions would send Putin a sobering signal that he will not be appeased.

Unfortunately, the Biden administration has so far refused to commit to any clear course of action. Although there is talk that lessons have been learned since Russia invaded Crimea in 2014, President Biden’s main course has been Zoom diplomacy and perpetual dialogue. Such an empty tack will never tame the Russian bear.

This moment did not emerge overnight. It is the culmination of Putin’s years-long campaign of information warfare and skirmishing, which has softened the ground for a full-on assault. Putin has been goading the West for years to find out where the line is. It now falls to President Biden as commander in chief to give him a clear answer.

President Biden may be tempted to concede limited autonomy to the Donbas to avoid a full Russian invasion. This would be a moral victory for Putin. It would erode Ukraine’s sovereignty and prove that Putin can bully his way to reassembling the Soviet empire. It would also reduce Ukraine’s chances of one day joining NATO, which it should feel at liberty to do.

Ukraine is not a treaty ally, but U.S. credibility is at stake nonetheless. A show of U.S. weakness in Ukraine would reverberate globally. It would invite Russia to continue its dangerous behavior across Eastern Europe, in the Baltics, and in outer space. And it would give China, our greatest adversary, reason to doubt American resolve and put our ally Taiwan in greater danger.

Clearly there is much on the line in Ukraine — for Europe, for America, and for the entire world. President Biden should take this opportunity to shore up the lagging trust of our allies and the waning respect of our foes. He has no time to lose.

ROGER WICKER, a ranking member on the Helsinki Commission and a member of the Senate Armed Services Committee, represents Mississippi in the U.S. Senate.

This op-ed originally appeared in National Review.