International Relations 101
There are no friendships or partnerships, or even enemies in international relations; there are only mutual or competing interests. If you expect states to operate with moral consistency, then you have to understand this: their own interests are their highest moral value, and they are absolutely consistent in their devotion to this.
No state can feel perfectly secure in its relationship with any other state, because interests can change. Every alliance between nations is, by definition, an alliance of convenience, not conviction. States are duplicitous, they are insincere, they are scheming, they have not one, not two, but many faces; and that is not a criticism; that is the nature of diplomacy and international relations.
Every government enters the game of global and regional politics with a limited set of assets; its geopolitical, geostrategic circumstances, its resources, its demographics, etc. To a great extent, the real power of any state is basically finite; it increases or decreases according to leverage. Any move a country makes may coincide with the interests of another country or contradict with them, this may give that country more, or less leverage.
When the base assets of a country change, that is extremely significant, it rarely happens, and almost never happens suddenly, but over the course of many years; such as demographic changes, or changes in industrial activity or in military power. When its base assets change, it impacts all of that country’s interconnections and alliances; not always positively.
A long-time ally of a powerful nation may become that country’s competitor if its assets become relatively stronger. And this is particularly true for any nation in its relationship with the United States.
You have to understand that, form the American perspective, total global hegemony is considered necessary for preserving its national interests. A country may become a regional power, and not a global power, but the US will still regard it as a threat to its security; not because the country can directly threaten the United States, but because it can potentially challenge American hegemony in a particular region, and no region must escape US dominance.
For the US, they would prefer for an entire region to sink into chaos than to become organized under the auspices of a regional power they do not control, or even, that they suspect they will not be able to control in the near future.