The U.S. military has had its difficulties with relatively minor wars. From Scott Ritter at sputnikglobe.com:

© AP Photo / Shakh Aivazov
Sputnik brings you the latest insights from Scott Ritter – former US Marine intel officer and UN weapons inspector – where he explains why the US Army is not ready for the full-scale war against Russia based on the failed Ukrainian counteroffensive attempt.
War is hell.
Before the Ukrainian conflict began, the US Army, drawing upon Cold War estimates, assessed in the 2019 edition of Field Manual (FM) 4-0 (Sustainment Operations) that US Army theater medical planners “may anticipate a sustained rate of roughly 3,600 casualties per day, ranging from those killed in action to those wounded in action or suffering disease or other non-battle injuries”, putting the US Army on track to lose some 50,000 casualties in two weeks of sustained combat operations against a Russian-style threat.
Are these numbers realistic? Ask Ukraine. In the lead up to the current counteroffensive, Ukraine built up three brigades-worth of troops (around 20,000 soldiers) along with another nine brigades (some 37,000 troops) trained and equipped by NATO, all of which were slated to participate in the main offensive effort in and around the village of Rabotino, in southern Zaporozhye. These forces were supplemented by an additional 40,000 territorial forces formed into eight so-called “shock brigades” intended to be deployed offensively in the vicinity of the city of Artemovsk (Bakhmut). The total number of Ukrainian troops mobilized and trained specifically for the counteroffensive was just under 100,000 men.
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