Why has the U.S. lost its gravitas among the nations of the earth?

The answer would be: Trump.

The last 2-3 administrations haven't helped.  Having said that, the world wants to put little, to no effort into doing any of these things themselves  But instead likes to *itch @ the US.  You don't like how we're doing things feel free to do them yourself.  On no, can't do that.  Better to sit on the sidelines & moan, whine & complain.    

It elected Trump and the rest of the world is baffled as to why.

it began in the 1990s, when the USA emerged as the sole global superpower and decided to throw a decade long orgy of vapid consumerism even as new democracies cried out for leadership and the spectre of genocide returned to the European continent....

things felt so good for so many and the boomer generation could finally stop hearing about all that "responsibility" stuff from the old folks and go about dedicating themselves to their own pleasure without guilt...

...the standards that built America into that superpower, quite simply, didn't matter anymore... and as the "me" generation begat the "mine" generation and then the "all about ME" generation, those standards dropped lower and lower...

...the fact that you will hear non-stop responses about "trump" and "libs" today being the WORST EVER this or that is simply a reflection of how self-absorption has become new American way..... whether you hate Trump or Pelosi and Co., the simple fact is that the all trotted into town over a bar that had been lowered decades ago....