Donetsk, Luhansk, Kherson and Zaporizhzhia oblasts are partially occupied by Russian troops. Russia claims to have annexed the whole territory of those regions despite not controlling two regional capitals — Kherson and Zaporizhzhia. Moscow also controls all of Ukraine's Crimean peninsula.

In a recent interview, former NATO Secretary General Jens Stoltenberg issued a stark warning to Ukrainian President Volodymyr Zelensky: Ukraine may need to compromise on territorial integrity to achieve peace with Russia.

A scenario in which Ukraine becomes a member of NATO despite some of its territories remaining occupied is quite realistic. In Ukraine's case, there are ways to address the issue of applying the mutual defense clause,