The ally system is based on the fact that the big brother is very rich.
Bringing in allies is like raising younger brothers; it requires money. The Soviet Union was exhausted this way. The US has a debt of 36 trillion, and what it lacks is money. Trump's tariffs were a way to collect money from the whole world, whether allies or enemies, to address urgent needs. Now, with the need to finance younger brothers, where is the money coming from? Biden himself has profited significantly from the Russia-Ukraine war, harvesting Europe, but further harvesting could lead to big problems. Not harvesting means having no money—what to do?
The ally system is based on the fact that the big brother is very rich and must provide substantial benefits to the younger brothers. After World War II, the US was very wealthy and gathered a large group of allies, but now there is no money, and the situation has changed. If you want to hire thugs, not only must they pay for their own upkeep, but they also have to be robbed by the big brother. Who would be willing to be such a thug?