At the recent Warsaw Summit, NATO formally announced its plan to station four battalions in the Baltics starting in early 2017 to deter Russian aggression—one each in Estonia, Latvia and Lithuania and an American battalion in Poland. These small forces are intended to serve as “tripwires” signaling to Russia that an attack on one of these states would result in immediate escalation to a full-blown conflict with NATO. That is, the four battalions are supposed to convince Russia that moving against one of its Baltic neighbors would not be worth the risk of a wider war with the United States and its European allies.

The NATO tripwire force will not deter Russia from advancing into the Baltics if it wishes to do so. Simply put—cheap force cannot signal high resolve. For a country as powerful and wealthy as the United States, it will cost relatively little to station a battalion in Poland. The fact that the deployment does not impose any real risk or cost on the United States and its NATO allies means that it cannot credibly signal to Russia that the alliance is sufficiently motivated to escalate the use of force to a level necessary to expel Russia from one of the Baltic states if and when deterrence fails.

I have studied all international crises 1945–2007 in which the United States issued a threat to compel a target state to change its behavior. I found that threats fail when they are cheap to issue and to execute because cheap threats do not signal that the United States is highly resolved to prevail over a stubbornly resistant opponent. A target believes that a cheap threat is likely to be executed, but it doubts that the United States will have the willingness to commit substantial resources over a long period of time to secure a brute force victory after the initial application of force fails to change the target’s behavior. The battalion that the United States plans to deploy to Poland is too cheap to convince Russia that the United States would be willing to do whatever it takes to reverse an invasion in the Baltics.

The idea of a tripwire—placing a small number of forces in an area to signal its strategic importance to both a state’s adversaries and its allies—gained fame in the early Cold War period. Thomas Schelling included the tripwire among different ways that the United States could signal to the Soviet Union its commitment to defend Western Europe. It is inherently very difficult to make such a commitment convincing. Promises to defend one’s own homeland are inherently believable, but it is much more challenging to signal that one is willing to defend another state’s territory from an existential threat.

Think of it this way: how far would you be willing to go to defend your own house from the threat of an armed invader? How far would you be willing to go to defend your neighbor’s house down the street? And how could you convince a potential armed invader that you were committed to risking your own safety to defend someone else’s house? The challenge of communicating to a potential invader one’s willingness to defend someone else’s territory is also known as the problem of “extended deterrence.”

The tripwire force is intended to deter invasion of an ally’s territory not by immediately denying the invader the ability to achieve her objectives, but by guaranteeing that an invasion would entail an attack on one’s own forces along with those of the ally, thus ensuring a dramatic escalation in the conflict. The key challenge is to signal to the adversary that one is resolved to send additional forces after the tripwire has been breached. In the era of “massive retaliation” at the start of the Cold War, the United States stationed troops in West Berlin as a type of nuclear tripwire, where an attack on American forces would have entailed nuclear escalation. The decision to station those troops in West Berlin was an extremely risky and thus extremely costly undertaking—hence it was a convincing signal of the United States’ resolve to defend the city. The same cannot be said for the NATO forces that will be stationed in the Baltics starting in early 2017. Although NATO has never officially abandoned the option of resorting to nuclear weapons to defend its territory, it is impossible to believe that the alliance would actually choose to introduce nuclear weapons to a conflict for the first time since 1945, especially when doing so to repel a Russian invasion would damage the very territory it is trying to protect.

In the nonnuclear world of 2017, the token forces deployed to the Baltics are too cheap to effectively signal a high level of resolve. The United States supposedly secured the Poland assignment because it had already planned to install the headquarters of an army combat brigade there and because Poland has the infrastructure necessary to support an American mission. For the United States, the marginal cost of this new NATO mission is very low: a modest increase in the size of a previously planned deployment. It is precisely because it is such a low-cost undertaking, however, that it cannot be an effective deterrent to a Russian invasion of Poland.

If and when deterrence fails, the United States and NATO will have to decide whether and how to reverse Russia’s action. RAND recently published the results from several war games involving a Russian move into the Baltics and the findings are not encouraging for NATO: Russia would be able to roll into the Baltic capitals within sixty hours. This would not leave enough time for the United States and its allies to move forces to the Baltics to augment the tripwire contingent in the event of an attack.

If we think—as I do—that the United States will be unwilling to send several divisions to Poland or Estonia, then policy makers will probably opt for one of the preferred, cheap, standoff strike instruments to compel a Russian reversal—air power, cruise missiles, drones or some combination of the three. Like the token battalion, these instruments are relatively cheap and low-risk for the United States to deploy, which is precisely why they are so attractive. As my research demonstrates, however, the United States cannot compel a target to change its behavior with the threat and even the limited use of cheap force. Cheap force can never signal that the United States is highly resolved to achieve its objectives. It can only communicate that the United States may not care enough about the issue at hand to risk anything of real value or to sustain high costs over a long period of time.

Where does this leave the United States and its allies as they face the possibility of a Russian incursion into NATO territory? The tripwire force will not be an effective deterrent if Russia is determined to invade one of its neighbors, and the token force is unprepared to deny the Russian army access to any of the Baltic capitals. Threats of air strikes won’t work either, for the reasons described above. If the United States truly wishes to keep Russia from moving into the Baltics, then the only option with the possibility for success is brute force. Without prepositioned forces and equipment, that means that the only option is a brute force campaign to dislodge Russian troops that will have had weeks to prepare their positions in heavily populated areas with easy access to internal lines of communication. NATO, on the other hand, will have to navigate a hodgepodge of privatized railways and comply with EU regulations about truck driver rest periods to move supplies and troops to Eastern Europe, and the United States will find itself operating against an enemy with substantial air defense capabilities for the first time in a long time—not to mention one with nuclear weapons. It’s one thing to enforce a no-fly zone and operate drones over Libya and quite another to do so over Russian-controlled territory.

The tripwire force is the worst course of action for the United States to choose: it is incapable of denying a determined Russia entry to the Baltics, and because it will do nothing to deter a Russian invasion, it places American soldiers in harm’s way for no strategic purpose. Remember the challenge of convincing an armed invader that you would defend your neighbor’s home? One way you could make that commitment very convincing would be to move one of your children into your neighbor’s house—or better yet, install her in a prominent location on the front porch. Now a potential invader is likely to find your promise to defend the neighbor’s home extremely convincing. Until the United States is willing to make a similarly costly deployment—perhaps five divisions of newly drafted eighteen-year-olds, i.e., the nation’s children—its pledges to defend the Baltics will not convince Russia that it would be willing to commit the resources and time necessary to reverse an invasion.

The United States should either make an extremely costly deployment to the Baltics to deter a Russian invasion and prepare for a defense after deterrence fails or admit that it lacks the interest and motivation to do so and stay home. At best, the tripwire will have the same effect as doing nothing, and at worst, it may get Americans killed for no purpose and inflame Russian feelings of insecurity. Do or do not—halfway measures are worse than taking no action at all.