MAY 2025
What Allies Want
European Priorities in a Contested Security Environment
By Nicholas Smith Adamopoulos
I
n recent years, several U.S. allies have expressed a growing interest in developing independent
nuclear programs. This trend could reect a combination of factors, such as geostrategic
uncertainty, growing regional tensions, and questions about the credibility of U.S. security
guarantees over time. The United States has dedicated signicant eort to preventing the spread of
nuclear weapons to new possessors for much of the nuclear age and has successfully utilized extended
deterrence to dissuade allies from succumbing to proliferation pressures. While U.S. allies have
expressed interest in developing their own nuclear weapons before, the United States has historically
gone to signicant lengths to prevent allied proliferation. Today, U.S. allies in Europe are once again
experiencing regional security pressures and fears of U.S. abandonment, both of which have
historically motivated interest in proliferation. Yet, by and large, allied leaders have still caveated
public preparations for a revised transatlantic relationship with the sentiment that the status quo of
cooperation with the United States is preferable to its alternatives.
To explore this phenomenon, CSIS’s Project on Nuclear Issues (PONI) gathered a range of views on
allied priorities and preferred outcomes for U.S. extended deterrence, modernization, and arms
control eorts across a series of workshops with both U.S. and European experts during the winter
of 202425. The workshops utilized an alternative futures methodoloy, developing four scenarios that
covered a range of adversary capabilities and cooperation along with variations in NATO cohesion.
Accompanying the four core scenarios were two “black swan” scenarios, one in which Russia used
nuclear weapons and another where there was a change of political leadership in Moscow. The
workshops captured strategic objectives and arms control priorities for U.S. allies across the scenarios,
providing a robust look at what U.S. allies want and why the current transatlantic relationship is a more
desirable outcome than other options.
While some allies feel compelled to consider alternatives to U.S. extended deterrence, they do not want
to pursue nuclear proliferation unless they feel there is no choice. Instead, allies prefer a continued
close relationship with the United States underpinned by the guarantee of U.S. extended nuclear
