Introduction At an early stage in his second term, US President Donald Trump initiated a fundamental shift in US foreign and secu­rity policy. He has sought to normalize relations with Russia and signaled his willingness to end the war in Ukraine on Russia’s terms. Trump has imposed tariffs on European al­lies, as was already the case during his first term. He did not even exclude the option of using military force to incorpo­rate Greenland and has repeatedly suggested that Canada should become the 51 st state of the USA. Profound changes in US foreign and security policy have been in the making for some time. Even for long-term observers of US policy, it is however astonishing to see how swiftly and radically Trump is steering America in a new direction. This policy shift will have profound, if not existential, conse­quences, for NATO. In terms of military resources alone, the US has been indispensable. In 2023 the US still accounted for almost 70 percent of all allied military spending(916 bil ­lion USD of 1340 billion USD in current prices)(Stockholm International Peace Research Institute 2024). When it comes to the combined figures for NATO allies’ armed forces, with 3.2 million men and women in uniform, the US contributes 1.3 million to the total(The Military Balance, Chapter Three: Europe 2024). 1 Beyond mere capacity, NATO allies remain very reliant on many US military capabilities and“strategic enablers” such as logistics, heavy lifting, intelligence, air de­fense, and so on. The domestic debate in the United States on key aspects of foreign and security policy is lagging behind the changes already implemented by Trump and his administration. For instance, many Republican members of Congress, who have been staunch supporters of NATO and Ukraine’s fight against Russian aggression, were seemingly caught off-guard by Trump’s new policy. After the fact, many were reluctant to raise concerns openly, or they even fell in line and now sup­port the new policy. Lawmakers from the Democratic side are raising concerns but are struggling to be heard and to form a coherent opposition. It is increasingly obvious that the US Congress will not be willing and able to act as a cor­rective to the new president’s impulses. This was different during Trump’s first presidency. The same gap between public debate and official policy is ev­ident with regard to think-tank publications. To be sure, voices critical of a strong US engagement in NATO, NATO enlarge­ment or military support for Ukraine had been heard before Trump moved into the Oval Office again. These voices had however been marginal. The end of Trump’s first term saw the establishment of new think tanks that advocate for policies in line with Trump’s views and his MAGA movement. These in­clude the America First Policy Institute and the Center for Re­newing America. It is very likely that they will now become more vocal and more dominant in the public debate. To date, think-tank discourse on NATO and NATO-related topics has been dominated by internationalist, Atlanticist, and deterrence-focused voices. Against the backdrop of Russia’s full-scale onslaught on Ukraine in February 2022, NATO’s new 2022 Strategic Concept, the Alliance’s 75 th ­an niversary summit in 2024, and with the tail wind from the Biden administration, most think-tank input on NATO strongly advocated for enhancing transatlantic defense co­operation – with a greater European contribution, of course – supporting Ukraine in its fight against Russia, and beef­ing-up the Alliance’s defense and deterrence capabilities on its eastern flank. It remains to be seen how the radical policy shift under “Trump II” will influence think-tank discourse. Some ana­lysts are already adapting their previously Atlanticist and deterrence-focused messaging to the new political circum­stances. They discuss how US engagement in NATO can be turned into“a new deal for America”(The Alphen Group 2025) and how a“new transatlantic bargain”(Hooker and Molot 2025, p. 17) can be found that will survive the second Trump administration and whatever comes after it. Others, such as Max Bergman from the Center for Strategic and In­ternational Studies(CSIS), are more skeptical that it will be possible to achieve a new deal, specifically one that in­cludes Europeans spending much more on defense and buying more US weapons or US liquefied natural gas (Bergmann 2025, p. 5). Strategic Outlook and the US National Security Debate For the past several decades, US foreign and security poli­cy has been based on the assumption that American secu­rity and prosperity depend on a deep political, economic, and military engagement with the rest of the world. To some extent, this view was even reflected in Trump’s Na­tional Security Strategy from his first term(2017-2021). Ad ­mittedly, this strategy – adopting the conceptual headline of“principled realism” – already signaled a departure from earlier US strategy documents. It portrayed the world in rather dark colors as an arena in which states and non­state actors compete for economic and military power and influence. According to the 2017 NSS, the major threats emanate from revisionist powers(China and Russia), rogue states(mainly Iran and North Korea), and jihadist terror ­ism. The authors of the strategy also complained that oth­er countries, including allies, have taken advantage of America’s generosity. At the same time, Trump’s first NSS still emphasized the importance of the international order and its institutions, which America helped to create after World War II. It also committed the US to NATO and its collective defense clause, including the extended nuclear deterrent provided by the US(President of the United States of America 2017). 1  The troop numbers refer to active-duty forces only. 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