there was that brief spat with Vietnam over Cambodia

NATOpedia says "nearly 400 military interventions" for the US, listing this as the source.

China's is likely referring to the Korean war or the Vietnam war. So it should probably be 2 ig, though for Korea China was firmly on the right side of history.

https://www.thenews.com.pk/print/595752-the-us-has-been-at-war-225-out-of-243-years-since-1776

https://en.wikipedia.org/wiki/List_of_wars_involving_the_People%27s_Republic_of_China

People have already answered, but consider also that the US has over 750 overseas military bases around the world, and is building more to further encircle China. Meanwhile China has one anti-piracy base in Djibouti.

Neither country is state-run capitalism. And anyway, state capitalism is an oxymoron when a state has fiat monetary sovereignty—which the US and China do. Such a state has no need to make a profit, which is foundational to capitalism, because it can and does create as much money as it pleases. You don’t have to take my word for it, you can take Alan Greenspan’s.

China is not state run capitalism, but I guess western libs will never stop repeating it in face of all evidence to the contrary.

And the respective political systems of these two countries obviously play no role in this.

Do you think this is because the leadership is magically nicer for some reason, or because their political/economic system is better able to stay out of conflict?

It also overlooks that China has historically been conquered by outside forces as its leadership grew weak. And the Communist party was having enough trouble internally feeding its people and causing issues like the "break four olds" period that there was little ability to wage effective war.

Let's also not forget the the communists literally aided the Japanese in the rape of Nanking as it helped to undermine the GuoMinDang.