While Russia is being heavily criticized by the West for its incursion into Ukraine, the Russian constitution does, in part, allow for this kind of action in a sovereign country.
Article 61, section 2 of the constitution allows defiance of international laws in this situation. It reads, in English:
"The Russian Federation shall guarantee its citizens defense and patronage beyond its boundaries."
This is the language the Russians evoked during the 2008 war with Georgia. And it's language that Russian Foreign Minister Sergey Lavrov referenced Monday, saying, "We are talking here about protection of our citizens and compatriots, about protection of the most fundamental of the human rights — the right to live, and nothing more."
Russian authorities take the language of Article 61 very seriously and uncompromisingly. In 2008, Valery Zorkin, the chief justice of Russia's constitutional court, penned an op-ed on how Article 61 justified the Georgia invasion. He wrote (translated, via Google Translate, emphasis mine), "The government and the president of Russia had no other way than in the strongest terms to ensure compliance with Article 61 of the Constitution, which in black and white that the Russian Federation shall guarantee its citizens protection and patronage abroad."