Russia has not invaded Ukraine...and the Crimean's were very happy to be annexed by Russia.
I suppose you think the USA does NOT enter nations by the force of arms.
I disagree with you on whether Russia invaded Crimea and whether Crimea was legally transferred to Russia. The international community, including the UN disagrees with you also. The Tatars, who have been there longer than both the Russians and Ukrainians disagree with you too. Even the Chinese refused to vote on the legitimacy of the sham election in Crimea. Many of the Crimean's that wanted to be annexed to Russia are already having buyers remorse. They are now suffering the second invasion. This one is not military, it is economic. The country is being overrun by "Moscow" carpetbaggers who are pushing many of the ethnic Russians who supported the annexation, to the side. By the way, Putin has admitted Russian troops and mercenaries participated in the take over of Crimea. The "Green" ,men running around armed with Russian weapons and armored vehicles, wearing Russian uniforms with the insignia's removed were Russian. The first mercenaries to take over the Ukrainian border post were member of a huge Russian so called motorcycle club or gang called the Wolves. They showed up well armed and backed up by Russian troops with tanks.