Honestly the only one that’s coming to mind is the 1st century AD Roman military. Not that even they didn’t suffer defeats on the battlefield.
Also technically not “history” but I have a hard time imagining anyone could come close to defeating the US today.
I would disagree on that last statement. We don’t have the infrastructure to sustain a high intensity, war of attrition against a peer nation situated on the other side of the globe like Russia or China. From a casualties point of view, war games and other simulations have such a war looking unnervingly like World War I, and naval strategists in particular are concerned that our inability to keep up with maintenance schedules for our ships and submarines is slowly giving China the edge in the Pacific. This is not to say we would be steam rolled if conflict were to break out, but I think the probability of defeat is greater than most people tend to think.
First one I thought of.