Membership of Ukraine in NATO is the red line for Russia. Russia should not allow Ukraine to become a member of the Alliance because Ukrainians and Russians are one nation. Membership of Ukraine in NATO will create conditions for civil war. The US is especially interested in it.
A recurring pro-Kremlin narrative about the all-Russian nation and the Russian world/civilisation. In this case, the narrative is used in the context of Ukraine's possible NATO membership.
Ukraine is a well-defined nation-state with a long history and its own literature and identity, despite foreign rule for long periods of time. Ukrainians and Russians belong to the Slavs ethnolinguistic group but they are not a single nation.
Membership in NATO will not create conditions for war between Ukraine and Russia. NATO is a defensive alliance and its purpose is to guarantee the freedom and security of its members through political and military means. Ukraine is a sovereign state, free to decide its political orientation incl. on international organisations and military alliances. Its relationship with NATO dates back to 1991.
Pro-Kremlin outlets often recur to the claims that Ukrainians and Russians (as well as Belarusians) are one nation. See similar cases here, here and here.