Jacques E. C. Hymans, Associate Professor of International Relations, University of Southern California

more possible for Russian President Vladimir Putin. Here as elsewhere, nationalist-populist rhetoric may be the effect of policies that are caused by more important causal factors: nationalist populism may be more effect than cause. Regarding President Donald Trump contradicting the 2019 US Missile Defense Review (pp. 14–15, 19), one can argue that such unpredictability can under some conditions rationally serve US interests. Regarding Defense Secretary James Mattis and General Mark Milley, the chairman of the Joint Chiefs of Staff, inserting themselves into the nuclear-weapons chain of command to prevent the worst excesses of President Trump (p. 15), can we confidently say that these excesses resided in Trump’s nationalist populism and not a range of other biases and idiosyncrasies? The finding that (nationalist?) populists are more likely to communicate via Twitter is fascinating, although whether this makes their threats and assurances more or less credible (and dangerous) is an open question. The claim that nationalist populists are more likely to damage institutions underpinning global order is also important. President Trump walking away from the Iranian nuclear deal in 2018 is cited as an example—as is President Putin’s annexation of Crimea and President Trump’s approach to NATO (pp. 17–18)—but here again, it is not clear that it is Trump’s nationalist populism doing the work. It would be interesting to see an analysis of all such policies over a longer period of time and the role of nationalist-populist leaders in causing them. Erdogan’s reliance on personal diplomacy to acquire a Russian missile-defense system (p. 19) is one case among many in which leaders prefer to meet, exchange information, and get to know other leaders personally, and the role of nationalist populism here needs to be further explored. Ultimately, Meier and Vieluf call for “more empirical data” and “specifics of nationalist populists’ policies in a more comprehensive and systematic manner” (p. 22). I hope that the comments in this response might serve as a starting point for that effort and help Meier and Vieluf and others continue this important and promising research.