“America is back. The transatlantic alliance is back.” That’s the big takeaway from President Biden’s recent speech to world leaders at the Munich Security Conference. These remarks echoed Biden’s previous efforts to reaffirm support for U.S. allies in NATO and elsewhere, and demonstrate a clear break with his predecessor, Donald Trump, who questioned the U.S. commitment to its alliances.

These gestures serve an important function within alliances, by reassuring allies that the United States is in fact still interested in their security. Here’s why this matters: This type of reassurance can dissuade allies from adopting policies that could reduce their commitment to the alliance, diminish U.S. influence or trigger regional arms races. Such policies might include pursuing nuclear weapons, rapprochement with U.S. adversaries, or forming alternative alliances. My research on alliances suggests that U.S. officials reassure allies with these concerns in mind.

What is reassurance?

Alliance treaties are among the most important ways countries commit themselves to each other’s defense. By putting this commitment in writing, countries can create reputational costs to reneging on their promises to support each other in times of need.

Reassurance is the practice of reminding allies that these initial promises still hold, and that the United States will provide the protection it has promised. Signals of reassurance reinforce alliance treaties by demonstrating that Washington is willing to commit resources on allies’ behalf, ensuring that the United States has military capabilities nearby to assist allies, and further staking U.S. reputation on defending allies.

How do policymakers signal this kind of reassurance? One tool, of course, is deploying U.S. forces to allied territory. This directly puts American lives at risk and provides capabilities that can be used in a crisis. But while troop levels in allied countries are typically stable in most years, they are also subject to rapid changes, leaving allies with concerns about future U.S. troop levels and intentions.

Other signals, such as high-profile diplomatic visits and leader statements, are more symbolic. Yet these gestures create reputational costs and thus credibly signal that the United States will remain committed to the alliance for the foreseeable future.

Biden has used a mix of these signals already, by canceling Trump’s plan to withdraw 12,000 U.S. troops from Germany and telling officials in allied countries that his administration is recommitting to U.S. alliances. This is consistent with the historical norm; since 1950, U.S. presidents have on average issued eight statements of reassurance to allies each year, and Washington has maintained over 100,000 troops in allied countries.

Why would the United States reassure its allies?

Allies have good reason to seek reassurance, as circumstances can change over time — new leaders come into power, for instance, or the threat environment shifts. Allies might worry that Washington could eventually terminate the alliance — or perhaps even renege on its promise when called to assist. The United States, for example, terminated its formal alliance with Taiwan in 1979, when the United States and China normalized diplomatic relations.

The United States also reassures its allies to discourage them from finding substitutes for the alliance. My research suggests that the United States is more likely to reassure allies that have greater ability to go their own way, whether by pursuing nuclear weapons, military self-reliance or alignment with third parties. Any of these decisions can reduce an ally’s dependence on U.S. protection. This, in turn, diminishes U.S. influence and future bargaining leverage and is likely to make allies less cooperative with U.S. preferences.

France, for example, left NATO’s integrated military command in 1966, several years after obtaining nuclear weapons, and only rejoined in 2009. And in response to Trump’s presidency, French President Emmanuel Macron expressed his desire for a European army that could allow Europe to become more independent of the United States and hedge against U.S. unreliability or bullying on cost-sharing. Biden’s commitment to NATO could blunt some of this momentum, though Macron has continued to argue that greater autonomy would insulate Europe from future U.S. policy shifts.

Do the benefits of reassurance outweigh the costs?

Reassurance is not without its critics, who argue that making allies too confident about U.S. protection carries its own risks. For one, allies might not invest heavily in the alliance’s defense capabilities, preferring to let Washington shoulder the burden — that’s a perennial, bipartisan concern among U.S. leaders.

Similarly, if allies are too confident in U.S. backing, they might be tempted to behave more aggressively toward third parties. Indeed, Trump justified withholding assurances of support by suggesting that allies would be unlikely to pay for their own defense otherwise and expressing concern that a “very aggressive” Montenegro could drag the United States into war.

Concerns about the downsides of reassurance are real. But leaders can often alleviate these negative effects in practice. U.S. leaders do not reassure at random, and the very fact that allies need reassuring means that they’re unlikely to have excessive confidence in U.S. protection. The allies most in need of reassurance are likely to be those in high-threat environments or those with doubts about U.S. commitment. These same allies are also likely to be more receptive to U.S. requests for greater burden-sharing.

Many allies that have historically hosted large numbers of U.S. military forces, such as South Korea and West Germany during the Cold War, also contributed a great deal toward common defense in the region. Moreover, although reassuring allies may in some cases embolden them to behave more aggressively in search of gains, it may also discourage them from initiating conflict or engaging in arms races because they feel insecure.

Reassuring allies is neither a silver bullet nor an unmitigated good. But as long as the United States hopes to maintain its alliances, U.S. leaders are likely to continue to view reassurance as a useful policy lever to further U.S. interests.

Brian Blankenship (@BrianDBlank) is an assistant professor of political science at the University of Miami and a Stanton Fellow at the Council on Foreign Relations. His research studies military alliances and U.S. foreign policy.