U.S.  and  Ukrainian soldiers take part in the “Rapid Trident 2016″ joint military exercises near Lviv, Ukraine, on July 4, 2016. (Ivan Bobersyy/European Pressphoto Agency)

After Jan. 20, President Donald Trump will juggle a number of important tensions with longstanding U.S. allies. For example, Trump’s tweets suggest rapprochement with Russia is a priority, yet NATO allies and the U.S. Congress will press for support to NATO allies, such as those in the Baltic region. In Asia, Trump has promised to take a strong line against Chinese assertiveness, but has also questioned whether Japan and South Korea are contributing enough to contain China.

So Trump faces this dilemma: how can he reassure allies while engaging adversaries? Our research suggests one possibility: arms transfers.

In our recent International Security article, we elaborate on the “patron’s dilemma” that arises when relationships with allies change. We explain the decades-old U.S. playbook, which calls for the United States to signal reassurance and deterrence via arms transfers, while improving ties with adversaries.

Will Trump follow this playbook? Arms transfers are particularly useful when geostrategic priorities are shifting. Here’s why we think Trump will use this foreign policy tool:

  1. Arms transfers accomplish two important goals: upgrading local deterrent capabilities and offsetting fears of abandonment. Allies are better equipped to defend themselves, thanks to U.S. weapons.
  2. Arms transfers typically don’t attract as much attention as U.S. military deployments or new treaty commitments. But they can enable the recipient to engage in aggressive foreign policies that the United States might not desire, especially if the arms transfers enable offensive operations. Experts call this worry “entrapment” — when an ally gets drawn into another country’s conflict.
  3. Arms transfers signal a different form of commitment than alliances. After all, treaty alliances tie hands by requiring allies to fight for one another in any future conflict. Such treaties are formalized security ties — and usually follow a politicized ratification process. These “pieces of paper” are inflexible, but do not necessarily require follow-up military investment.

What sorts of arms transfers are most likely?

Two factors shape what combination of alliances and arms the United States might offer its clients.

First, U.S. foreign policymakers look at shared security interests when deciding whether to form or maintain an alliance. To make this determination, U.S. policymakers typically ask whether the client state has a similar or different ranking of security threats than that of the United States.

Here’s an example. Both Western Europe and the United States shared the view that the Soviet Union was the leading threat to their vital interests after World War II, so they formed NATO.

If the threat rankings align, Washington seeks to establish or strengthen an alliance. If the threat rankings differ, then U.S. foreign policymakers find that avoiding or downgrading an alliance commitment better serves U.S. interests. Indeed, that is why the Carter administration scrapped the U.S. formal alliance with Taiwan to normalize ties with China in 1978.

Second, U.S. leaders decide whether to arm the client. In making this decision, policymakers ask whether the client can deter its adversary without U.S. assistance. If the local security balance favors an adversary, then transfers of U.S. weapons may be necessary to bolster the client’s security. If the local security balance favors the ally, substantial arms transfers are unnecessary — and can even encourage entrapment by the client.

Here’s what this will mean in practice — more arms transfers

So how might the next administration approach the geopolitical conundrums associated with adjusting U.S. alliances? If U.S. leaders seek rapprochement with Russia, is there a quid pro quo?

A close aide of Vladimir Putin has already suggested that pulling back NATO forces from the Baltic region would reassure Moscow that Washington seeks closer ties. Yet if Trump obliges, then he might be under pressure from Congress and from NATO allies to reassure Baltic countries that the United States will minimize the risks of Russia exploiting any great power bargain.

To solidify the U.S. long-term commitment, Washington might offer lethal defensive weapons to boost Baltic ground and air defenses. This move would then enhance local deterrence.

Russia might have no other choice than to accept this compromise if its desire for rapprochement is sincere. For a precedent, look no further than the recent Iran deal. Since the July 2015 Iran nuclear deal, Saudi Arabia has received large amounts of U.S. weapons, including an arms agreement worth $1 billion, to attenuate its security concerns.

This scenario played out often after the normalization of relations with China. Washington agreed to recognize Beijing as the official government of China. Yet Washington often made major arms transfers like fighter jets and surface-to-air missile systems to Taiwan to reassure Taipei that the United States remained committed to Taiwan’s ability to defend itself against mainland China.

Following Obama’s footsteps?

Unlike his stance on Russia, Trump appears to favor a more hawkish approach toward China, as evidence by his comments on the one-China policy. Yet  Trump’s comments about burden-sharing by Japan and South Korea create a tension because balancing against China would require strengthening — not weakening — alliances with Japan and South Korea.

History suggests that more arms sales to Taiwan are possible because the cross-strait military balance is worsening due to China’s military modernization. The Trump administration might feel it necessary to make larger, more advanced, and more sustained arms packages than those committed to by President  Obama in 2015.

Moreover, despite calls for greater alliance burden-sharing by Japan and South Korea, the new Trump administration might invest more in those alliances to maintain local military balances against China and North Korea, respectively.

In short, the Trump administration might end up relying on arms transfers as its chief tool of reassurance and deterrence from Europe to Asia. In doing so, Trump would follow in the footsteps of the Obama administration, which authorized more arms sales than any U.S. administration since World War II. For alliance politics, arms transfers might be the new art of the deal.

Keren Yarhi-Milo is an assistant professor of politics and international affairs at the Woodrow Wilson School of Public and International Affairs at Princeton University.

Alexander Lanoszka is a lecturer in the Department of International Politics at City, University of London. Find him on Twitter at @ALanoszka.

Zack Cooper is a fellow with the Center for Strategic and International Studies. Find him on Twitter at @ZackCooper.