The use of the “red line” as a diplomatic ultimatum has a tarnished reputation after President Obama declared one to deter chemical weapons attacks in Syria, but then failed to enforce it when those weapons were used. Is the Biden Administration now taking enforcement more seriously and what would that mean?

Concern is mounting after what appears to have been a contentious meeting between Blinken and China’s top diplomat, Wang Yi, on the sidelines of the Munich Security Conference over the weekend. The United States pursued China for the meeting, hoping to restore relations after the spy balloon episode.

Mr. Wang eventually conceded the meeting, but China boasted in a subsequent public reading that the US had sought the bilateral session. The implication of the United States as supplicant was clear. Blinken later said China had not apologized for sending the balloon over US airspace and military sites.

Mr Wang reinforced the point in his public comments in Munich, chiding the US for what he called an “absurd and hysterical” response to the balloon. He also blamed the United States for being an obstacle to peace in Ukraine. Mr. Wang was heading to Moscow after Munich even as Mr. Biden was heading to kyiv.

The United States says China has provided Russia with technical and financial support for the war, but so far no weapons. Blinken’s alarm about the prospect is justified because it would exacerbate the conflict, increase bloodshed and make it more difficult for Ukraine to recapture occupied territory. It would also extend the war, further depleting the West’s arms reserves already stretched to the limit after a year of backing Ukraine.

To put it bluntly, arming Russia would be a new and explicit demonstration of China’s hostile intentions toward the United States and the West. It would certainly erase Beijing’s apparent desire since Biden’s meeting with President Xi Jinping in Bali late last year to improve US-China relations. It would also require a strong response from the US, which should include further economic decoupling.

But Xi and his war hawks may be willing to take that risk if they want to avoid a Russian defeat in Ukraine. China may want to bleed the West of its weapons and see if Russia can survive political support in Washington and European capitals for Ukraine. This would be silly and bad for China and the world, but the possibility is a more urgent reason for members of both parties in Congress to get serious about rebuilding America’s defenses.