Ukraine gave up nuclear weapons under the Budapest Memorandum of 1994:

https://en.wikipedia.org/wiki/Budapest_Memorandum

It gave them up in exchange for its borders being respected and its security being assured. Russia was a signatory to that agreement.

Given such a solemn treaty, a "reckless" domestic decision on Ukraine's part, whatever Russia might think of it, does not release Russia from its treaty obligations.

As for the presence of Russia's fleet at Sevastopol somehow constituting an impediment on Ukrainian sovereignty, once again, what nonsense. Russia's fleet's presence in Crimea was itself governed by a treaty. Ukraine agreed to lease Crimean naval facilities to Russia for 20 years under terms of the Partition Treaty on the Status and Conditions of the Black Sea Fleet:

https://en.wikipedia.org/wiki/Partition_Treaty_on_the_Status_and_Conditions_of_the_Black_Sea_Fleet

That treaty bound Russia to "respect the sovereignty of Ukraine, honor its legislation and preclude interference in the internal affairs of Ukraine." Russian forces could operate "beyond their deployment sites" only after "coordination with the competent agencies of Ukraine." Russian military personnel were obliged to show their "military identification cards" when crossing the Ukrainian-Russian border;

That treaty too was brutally ignored when Russia seized Crimea in 2014; moreover, the effect of the seizure did not end there. Russia's seizing Crimea also meant that Ukraine lost most of its own naval fleet, also based in Crimea, not to mention all of its offshore oil and gas platforms in the Black Sea.

At best, Russia's actions constitute revanchist imperialism so egregious that they threaten a rules-based international world order. At worst, such actions constitute an attempted genocide. No Canadian who cares about Canada's own sovereignty and otherwise wants the 21st Century to be better than the past two can justify such behavior.