How the global political landscape might change in a post-growth world is the stuff of thousands of speculative fiction and cinema plots.

You're looking outward at multinational alliances based on today's (mostly) nation-states. Another consideration is how those states themselves might fare. It strikes me as quite possible that larger states (the US, China, India, Indonesia) might well fragment, and even mid-sized powers (Spain, the Netherlands, Mexico) could splinter. The political situation in the US has been described as a "cold civil war" for some years (see: <https://www.aljazeera.com/opinions/2021/3/29/a-cold-civil-wa...>). Russia has been fighting to retain or regain ceded Soviet territories since the mid-1990s. There are separatist movements of various shades in Spain (Catalonia, Basque region), Belgium (Waloonia), the UK (Scotland, Northern Ireland), Canada (Quebec), India (multiple), Indonesia (multiple), Israel (Palestine), the Philippines (multiple), just as a list of more developed and stable nations. (I'm omitting Africa entirely, a huge list of itself, most of the Middle East, and Central and East Asia, largely as those deviate from the Western / OECD conditions fairly markedly.) China is of course unified, but has ethnic strife (Uygers and Tibetians), reintegrated regions (Hong Kong), and contested territories (Taiwain, Arunachal Pradesh, South China Sea, ...).

There've been outright separations: Czechoslovakia into the Czech Republic and Slovakia, the former Yugoslavia into Bosnia and Herzegovina, Croatia, Kosovo, Montenegro, North Macedonia, and Serbia (Balkans gonna Balkanize...). And reunifications, most especially of Germany.

The map of Europe has hardly been constant, and even as recently as the mid-19th century is largely unrecognisable today: <https://yewtu.be/watch?v=P9YnYRk8_kE>

Another factor to consider is that the same trends which would likely lead to degrowth will also make massive military campaigns far less viable. This affects not just the field of battle, but the entire logistical pipeline as well as the capability to build and resupply weapons, vehicles, ships, and ammunition. I'm reminded that the introduction of the sweet potato to New Zealand utterly reshaped that region's tribal landscape: the cultures with potatoes could march and campaign further than those without. (If I recall, this is discussed in one of Jared Diamond's books.) More recently, introduction of muskets to the Maori led to another technological-superiority disruption: <https://en.wikipedia.org/wiki/Musket_Wars>. Reintroduction of horses and the introduction of firearms to Native American cultures had similar effects.

Other thoughts:

- NATO exists not only as a bulwark against the USSR / Russia, but to protect access to Middle East oil and gas on which Europe depends in the extreme. Falling significance of both could alter that calculus.

- International shipping relies on safe seas and ports. Transoceanic trade blossomed under the British Navy and has flourished under American naval protection. Piracy exists only in small backwaters now (notably Somalia, though small-craft boardings are not unheard of particularly in central and Latin America, and S.E. Asia), and overt shows of force are not exceedingly common, but there are regular patrols off the Horn of Africa and Gulf of Aden, that I'm aware of. Rogue states including Iran and North Korea have attacked or commandeered vessels. Much as with shooting down commercial airliners, this is immensely disruptive to trade, and can result both in wide diversions of shipping routes and cessation of trade to unfriendly ports. Submarine warfare by both Axis and Allied forces was devastating during WWII, and both costs and countermeasures were extensive. NATO plays a role here as well.