The second reason is that Nato membership is popular among Americans. A Pew survey conducted in 2025 showed that 66% of US respondents thought that America benefited from Nato membership while 32% thought the opposite. While, as in many things, the US is divided – with more Democrat voters (77%) supporting Nato membership than Republicans (45%) – it’s clear that, on the whole, Americans approve of Nato membership.
The third reason is that leaving Nato would significantly weaken the US militarily. More than half a century of research by historians and international relations specialists has concluded that leaving Nato would also significantly weaken the US.
In 1989, historian Paul Kennedy’s detailed study of wars over a period of 500 years, The Rise and Fall of the Great Powers, found that a decisive feature of success in war is the resources that parties to the conflict can mobilise. Kennedy cites the examples of the two world wars and demonstrates that a key reason why Germany was defeated was that the allies could mobilise many more resources in manpower, arms production and economic assets than Germany and its allies. Eventually, this proved decisive in both conflicts.
Research into quantifying the military capacity of nations has been conducted for more than half a century as part of the Correlates of War project founded in 1963 by American political scientist J. David Singer. The project aims to systematically collect data about the causes and consequences of wars.
One of the datasets collected in the project is called the Composite Index of National Capability. This combines data on the demographic, industrial, economic and military capabilities of nations, including the US and China. The higher the index score the more resources a county has to fight wars.
The chart shows the size of the index for the top countries in the database. China is the most powerful nation in the chart with a score of 23 on the index. The US comes a rather distant second with a score of 13.