There's always the chance of a US - Russia confrontation over the Balkans in the 1990s. The Pristina Airport incident is usually the go-to POD for that sort of thing.
I can't find it anywhere, but I recall an article that said that Bush was seriously mulling launching airstrikes against invading Russian troops in Georgia back in 2008. Have no way of knowing if it's true or not, but...it's something.
More recently, we have the US-Russia situation in Syria. A couple PODs for this one, though it might be too recent. Back in October, the Obama administration was considering launching airstrikes against Syrian government targets even as Russia threated to retaliate for any such attack. And the Sharyat strikes in April could have been much, much worse had President Trump chosen to launch a saturation strike (a mass attack on Syrian military targets that would have almost certainly killed a few Russians - Defense Secretary Mattis had specifically cautioned against a saturation strike for this very reason) or had failed to inform the Russian military of their plans to strike Sharyat.
As for China, there was that Taiwan Strait Crisis back in 1996. Maybe the Chinese do decide to invade and the US intervenes to defend Taiwan.