Naw, that would have prevented us from toppling Sadam in Iraq, or Gaddafi in Libya. We couldn't support Saudi Arabia's current War in Yemen.
Better to be realistic: reward our friends, punish our enemies.
Ukraine? Friend.
Russia? Enemy.
It's simpler this way.
It would have stopped us from what is nearly universally considered a horrible mistake that led to immense bloodshed?
Why do you say that like a bad thing?
Like, the US at least attempted to invent a casus belli for Iraq, and it was one that Saddam couldn't aggressively contest without revealing Iraq's weakness to Iran. Gaddafi was deposed because his administration killed protestors and provoked a civil war. Yemen is a proxy conflict against Iran and while our involvement is indefensible the goal of the aggressors isn't to annex the country.
Even Russia has benefited from this ambiguity: the international response to the Russian seizure of Crimea was very limited, and there was essentially no organized sanction over their campaign in Syria (which included at least one attack on US soldiers).
In this conflict, Putin's speech ahead of the invasion of Ukraine made it absolutely clear that there is nothing provoking this conflict beyond Russian imperialism. The erroneously published document outlining the rationale assuming a Russian victory confirmed those sentiments [1].
Ukrainian resistance bought time for a coordinated response and the personal heroism and charisma of Zelenskyy (along with strong American leadership in the background) galvanized Europe. A hard line against wars of conquest is a good one to draw and rally around, even if it isn't perfect.
[1]: https://www.businessinsider.com/russian-state-news-published...
Again: reward your friends, punish your enemies.
Lie ("invent a casus belli") if you have to in order to punish your enemies.
Everyone here trying to give some ideological reason why we did such and such in this case, but not in this other case, is entirely mistaken. Literally everything is spin to justify the core motivation: reward your friends, punish your enemies.
Ukraine is no different. We can invade Iraq because they are our enemy. Russia CANNOT invade Ukraine because they are our friend.
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Answering your follow up here (I'm rate-limited):
> What was different about the 2014 invasion if it is strictly realpolitik friends and enemies?
As I understand things, Crimea is literally the friend/enemy distinction in action (Russian perspective):
Crimea, arguably (and certainly, effectively) was/is a Russian territory, autonomous (but pro-Russian) and with an active Russian naval base, that came under the nominal control of the West after the 2014 regime change (enemy); thus Russia militarily took it back (friend), ensuring its continued independence and pro-Russian stance.
Ukraine responded to Crimea's post-2014 autonomy/invasion/rescue/independence by cutting off their water supply, something Russia built for Crimea during the USSR. (I'm honestly surprised this step wasn't/isn't a Hague-level human rights violation. Maybe the Hague is also operating primarily under the friend/enemy distinction?)
More detail here: https://www.youtube.com/watch?v=If61baWF4GE (Why Russia is Invading Ukraine)
Russia was able to invade Ukraine and annex a big chunk of its territory in 2014, and the idea that Ukraine was less our "friend" then than it is now doesn't hold water to me. What was different about the 2014 invasion if it is strictly realpolitik friends and enemies?
Was in response to a US-supported coup against a president friendly to Russia. What are a US Ambassador and a US Assistant Secretary of State doing at a political protest in a foreign country?[1]
> U.S. Assistant Secretary of State for European and Eurasian Affairs Victoria Nuland arrived on Wednesday morning at Independence Square in Kyiv and began speaking with protesters rallying in support for Ukraine's integration with the European Union, an Interfax correspondent reported from the scene.
> Nuland is treating protesters with cookies, biscuits and bread from a big package. She is being accompanied by U.S. Ambassador to Ukraine Geoffrey Pyatt. Demonstrators are cheering Nuland with joyful shouts, some are chanting "God bless you."
> Nuland also approached Interior Forces troops deployed at the square and also offered them cookies, which the soldiers accepted silently.
[1] U.S. Assistant Secretary of State Nuland arrives at opposition rally in Kyiv (https://web.archive.org/web/20220228205358/https://en.interf...)
None of that, of course, is responsive to the original question of why Ukraine wasn't a friend in 2014 (in your narrative, this follows a coup and the installation of a friendly, western regime) but seems to be a friend now. Why Russia was able/allowed to annex Crimea with minimal international outcry is obvious—he either had or successfully fabricated a plausible enough claim—but doesn't fit in with the idea that all rules of conduct are applied only against enemies and not to friends.