Just like the current sanctions regime, which signals a Western unity in the face of Russian revanchism, the delivery of anti-tank weaponry to Kiev would signal America’s commitment to the post-Cold War European order and its international norms, which Moscow continues to threaten.
In other words, there is no compelling military reason to do this, and no U.S. security interests are advanced by it, but it sends a “signal.” That is not a good enough reason to provide weapons to one side in an ongoing conflict. As the Post reported last week, sending the Ukrainian government anti-tank missiles doesn’t make much sense :
But it remains unclear what, if anything, the delivery of an unknown number of Javelins could do to alter a battle that has mostly been relegated to artillery bombardment and nighttime skirmishes in no man’s land.
“This idea doesn’t flow from a policy or strategy” and may point to a political decision rather than military necessity, said Michael Kofman, an expert on the Ukrainian conflict and a senior fellow at the Wilson Center, a Washington think tank.
The only reason to provide these weapons is if the U.S. wants to encourage Ukraine’s government to go on the offensive. Leonid Bershidsky explained  this last week:
Two years after both sides have largely kept to existing demarcation lines (minor encroachments aside), it is militarily unnecessary to provide Ukraine with lethal weapons unless the U.S. wants to encourage it to try to reclaim the “people’s republics.” [bold mine-DL] That would be a mistake. Though Russia doesn’t have enough resources to take over and hold Ukraine while still staying on the lookout for other military threats, it has plenty of money, firepower and determination to defend the separatist statelets. Giving them up would mean the end of Putin’s aura of invincibility, leaving him vulnerable at home and overseas.
Charles Kupchan made  a similar point about this yesterday:
But the result would likely be the opposite — an escalation in the conflict that would lead to further losses of Ukraine’s territory and compromise its political stability. Russia enjoys insurmountable military superiority over Ukraine. The United States should not encourage Ukraine to engage in an escalatory confrontation with Russia. Washington knows full well that Ukraine cannot prevail.
While supporters of arming Ukraine want to send a “signal” of commitment to European order by “raising the cost” for Russia (i.e., killing Russians), the effect would be to cause more instability and violence mostly at Ukraine’s expense. If this is what constitutes “help” for Ukraine, Ukraine is better off without it. On top of that, our major European allies are opposed to this option, because they fear the escalation of the conflict that would likely follow from it. It is more than a little ridiculous for the U.S. to take actions in defense of “European order” that most Europeans oppose. Far from demonstrating “Western unity,” sending arms to Ukraine would highlight sharp disagreements within the alliance about how to respond to the conflict. Indeed, what unity there is on Ukraine could be jeopardized if the U.S. went ahead over the objections of our allies. Kupchan comments on this as well:
Europeans are already on edge due to Congress’s recent sanctions legislation, which imposes measures not coordinated with the European Union and that have the potential to cause undue harm to European companies. If Washington decides to head off on its own and send lethal weapons to Ukraine, solidarity on Ukraine may well come to end.
There is also potential danger for the U.S. and its allies in doing this:
Russia’s response to scattering Javelins among Ukrainian ground forces should factor into the decision, Kofman said.
“The Russians have a very clear policy of reciprocity, as we saw in the recent diplomatic purge. They see this as a premise of the U.S. wanting to kill Russians,” Kofman said.
“The answer to this won’t come in Ukraine.”
If the U.S. sends weapons with the intent of “raising the cost” for Russia in Ukraine (i.e., killing their soldiers and proxies), Russia could do the same thing to endanger U.S. forces in Syria, Afghanistan, or elsewhere. If the U.S. is going to risk that sort of reaction, there has to be a much more compelling reason than sending a “signal.” There isn’t one, and that’s why arming Ukraine  would be an act of stupendous folly.