Fundamentally I would say yes, however most of the countries that the US would likely do this to (Russia, Iran, China, Cuba) have contingencies in place through the implementation of country wide intranets[0][1]. Within those countries you also have localized social, financial and e-commerce services that minimize the general populations dependency on foreign companies and services.

So the US could cut cut them off or manipulate their connectivity to the global internet through something like BGP or DNS, but the impact would be far less than the US doing that against a country such as Canada that has a deep reliance on US infrastructure and services.

[0] https://www.theguardian.com/world/2019/feb/12/great-firewall... [1] https://en.wikipedia.org/wiki/National_intranet