That's funny as the thread is about contrasting Russia with US, but I don't seen to recall US invading any neighboring countries under the false pretense of defending some minorities.
But, seriously, if you remove "neighboring" from your statement (which seems to be not particularly relevant to a discussion of morality and respecting the sovereignty of other nations), I'm sure we can find plenty of bad examples perpetrated by the US, many of them much more recent than the Mexican-American War.
I remember the late Colin Powell brandishing proof of WMD in Iraq at the UN Counsel. He will never land in the TPI, neither will the UN Counsel for basing a judgment upon unverified allegation causing dozens of thousands of illegal killings by the US army.