Theoretically they could make those arguments now, but that's not the analogy.
The Analogy would Russia staging a Coup in Mexico, using psychopathic murderous Nazis, who hate America and Americans. They would begin funding pro-Nazi propaganda in Mexico's Textbooks, making the use of the English language illegal, and outlawing any political parties that aren't adequately anti-American enough. Additionally they'd begin conducting warfare against civilians in Baja and Sonora that speak English, whether American or Mexican by birth.
Wouldn't people in those border areas have the right want to leave an illegally changed Mexican Government, run by Nazis that want them dead? In fact, those English speakers organize self-defense militias to defend against the Russian-backed Nazis, trying to kill them, for not being Mexican enough, and too American. Wouldn't they have the right to hold referendums to leave Mexico, and become sovereign and independent*?
In those circumstances - wouldn't America be entitled to arm the separatist Mexican areas, to help defend themselves from Mexican aggression? If Mexico began building an Army, using Russian money and weapons, and repeatedly violating a series of peace accords, with the Separatists, in an escalatory manner...and missing troops in preparation for an offensive, would America not intervene? What if Russia planned to put Nuclear weapons there, and had been writing manifestos for Global Empire, with plans to use Mexico against the USA, and to separate America from S. America?
*There's no direct analogue for Crimea - which has always been Russian, or independent and autonomous, since the time of Peter the Great, and has had a Russian Naval base for hundreds of years. Crimea was handed back to the Ukrainian Socialist Republic, when it was united with Russia in the USSR, as gesture of goodwill, and perhaps as a sop to Ukrainian Nationalist/Nazis at that time in the 1950's, by Nikita Khrushchev the Soviet Premier. He didn't consult with anyone in Russia or Crimea, or Ukraine before he did that - and he was also a Ukrainian. In 1991, they were allowed an autonomous status, and their own legislative body, which maintained its legitimacy, after the Coup undermined that of Kiev's. After watching more than 50 Eastern Ukrainians murdered in nearby Odessa, but Ukrainian Nazis, while the police just stood by - they decided they wanted to rejoin Russia. They voted to do so - and the people in Crimea did as well, overwhelmingly.
