In any measure of global per capita rates of violent deaths, or military spending as a fraction of global GDP, we are currently in a golden age. Certainly, across the millennia. Arguably, post industrialization.
If peace means quietly sitting in the corner while havoc is wrought around you, then no, America isn't very peaceful.
If you consider the US to be an empire, then the statement is legitimately true. In terms of modern era “empires” it could only be compared to China or Russia, and the US is certainly more peaceful than those countries. If you want to look back through history, then all 3 of those countries are more peaceful than any empire that’s come before. The Roman Empire was kicked off by defeating Carthage twice, making them pay war reparations for 50 years, and once that was done going back and erasing their entire society from existence just to make sure they were never a threat again.
So is “peaceful country”. The ongoing existence of every country is predicated on its ability to wage war (or make friends with countries that wage war). Nearly all of the borders on the globe were defined by war. But that doesn’t mean countries or empires can’t have differing views towards peace and aggression.
In regards to China, I said it was less peaceful, which seems obviously true to me. China has ongoing territorial disputes with ever country close to it, which extends to raising man-made islands out of the sea to push back the sea borders of its neighbors. It believes it outright owns two neighboring countries, occupying one of them, and constantly threatening to conquer the other. A large amount of its northern region used to be Mongolia until the government one day sent millions of Chinese people to live there, and just redrew its maps. As a country it was also founded upon a revolution that included arguably the most brutal mass slaying of civilians in history.
The US fights more wars, but China is far more belligerent imo, and engages in outright territorial conquest every opportunity it gets.
If peace means quietly sitting in the corner while havoc is wrought around you, then no, America isn't very peaceful.