Last week, the Biden administration announced that it is freezing President Donald Trump’s decision to move U.S. soldiers out of Germany. Sebastian Schmidt is an assistant professor in Johns Hopkins Department of Political Science and an expert on the politics of overseas military bases. His new book, “Armed Guests: Territorial Sovereignty and Foreign Military Basing,” explains how such bases came into being in the first place. I asked him to put Trump’s and Biden’s decisions in historical context.

Henry Farrell: Your book describes how countries used to think that basing soldiers overseas was incompatible with state sovereignty. Why was this so and how did it change?

Sebastian Schmidt: Until World War II, a foreign military presence in a country was more or less the same thing as occupation by a foreign power. When a foreign power based soldiers on another country’s territory, it was because the country was a colony or a formal protectorate that the foreign power had some authority over. For example, America established its military base in Guantánamo Bay, Cuba, in 1903 under a treaty that allowed the United States to intervene in domestic politics in Cuba, which it later did. The United States and Cuba were not equals under this agreement.

This began to change in the period around World War II. New technologies such as long-range bombers and mechanized forces sped up warfare, and made it easier at long distance. Protecting against these new threats required “defense in depth” — a defensive perimeter which might extend far beyond a country’s border. When the Cold War began, policymakers in the West saw the conflict with the U.S.S.R. and its allies as an ideological conflict, where Western countries weren’t defending their narrow national interests, but instead protecting a shared transnational way of life against a different transnational system. In this context, it might not seem so bad for a country to have foreign forces on its territory. Finally, the notion that a state’s territory couldn’t be violated by other states was becoming increasingly accepted in international law and international norms. This eased fears that foreign military forces might end up occupying or annexing territory. These all came together to make foreign bases more attractive, and created the conditions under which states could cooperate to set them up.

HF: You describe today’s military bases as the product of a collaborative relationship between the state sending the troops and the state hosting them. What precedent do we have for one state unilaterally deciding to drop the collaboration, as Trump did in Germany?

SS: This has certainly happened, even between the United States and host states that continued to be close allies in other ways. One good example was when U.S. bases were closed in the Philippines in 1991 after the Philippine Senate did not ratify a basing agreement. At the height of the Cold War, French President Charles de Gaulle required the United States to withdraw all military personnel from France in 1967. This was no small request: France was home to important NATO infrastructure as well as 25,000 U.S. troops.

HF: What were the consequences of Trump’s decision for European states’ view of America?

SS: Some officials, particularly in Germany, worried that the move would make it harder to preserve the transatlantic partnership, especially given rising anti-American sentiments in Germany and elsewhere in Europe. The Pentagon framed the policy shift as strengthening NATO, but Trump’s own comments suggested that Germany was being singled out for punishment as an ally that had allegedly fleeced the United States for decades and was pursuing unfair trade policies. Other states, particularly Poland and the Baltic countries, were concerned that the U.S. was reducing the overall number of troops in Europe, but happy that some of the troops in Germany might be redeployed to their own countries.

HF: How are Germany and other states likely to view Biden’s decision to freeze the withdrawal of troops?

SS: Nobody likes unilateral moves, especially when they directly affect your security. However the U.S. eventually decides to restructure its presence, German officials will greatly appreciate the freeze and the Biden administration’s explicit willingness to work with Germany on this point. While some Germans don’t want a continued U.S. military presence, most elites of the major political parties do. Biden’s shift away from Trump’s policy signals a return to a more familiar role of the United States in Europe. There will still be important tensions between Europe and the U.S. [President Barack] Obama, too, described the Europeans as free-riders, and there have always been disagreements over who pays for NATO. The difference, however is that the Biden administration is willing to collaborate and negotiate, rather than make unannounced unilateral decisions as the Trump administration did.

HF: The new administration is undertaking a major review of U.S. forces overseas. What is the likelihood of substantial changes to U.S. basing practices?

SS: Big changes are unlikely. Trump’s regular threats to reduce America’s overseas presence — both to avoid being taken advantage of by allies and to bring an end to America’s “endless wars” — made him an outlier among postwar U.S. presidents. Yet despite his repeated and strongly worded statements, the actual policies only changed at the margins, and then mostly in conflict zones.

What is interesting is that Trump’s unilateralism and constant criticism of allies did not lead to basing relationships getting cut off. The reality is that states understand long-term, peacetime military presences to be an integral part of the management of security relations today. And the United States is not the only state that sends its soldiers to bases in other countries. Other countries, like France, Turkey, the United Arab Emirates, Russia and most notably China have bases abroad as well. Foreign bases are an accepted part of how states “do” security now, and significant changes would go against what has become more or less a foundational practice in security politics.