The Economist is cagey about the definition, but by context it works out to "America and its allies" (where "America" is "The United States of America".
The US, NATO, NORAD, ANZUS, SEATO, and specific alliances such as the US-Japan alliance, Mutual Defense Treaty between the United States and the Republic of Korea, and the like, would likely be included. In the context of Ukraine and this article, probably the Common Security and Defence Policy (CDSP) of the EU as well.
See e.g., "How the West should respond to China’s search for foreign outposts" (https://www.economist.com/leaders/2022/05/07/how-the-west-sh...), which uses the phrase "America and its allies" three times.
The US, NATO, NORAD, ANZUS, SEATO, and specific alliances such as the US-Japan alliance, Mutual Defense Treaty between the United States and the Republic of Korea, and the like, would likely be included. In the context of Ukraine and this article, probably the Common Security and Defence Policy (CDSP) of the EU as well.