Ok, what is the rest? Why did Putin invade Ukraine?
Many reasons. Why it is happening now is primarily because Ukraine, meaning the Ukrainian people, were steadily drifting out of Russia's orbit. No longer content to have a pro-Russian puppet as president, and increasingly leaning towards Western Europe. A 12 year study prior to the first invasion in 2014 revealed negligible friction between Ukrainian and Russian speakers, even in Crimea and the Donbas. The Maidan revolt was the spark point of the whole debacle, the first move that really made it clear to Putin Ukraine was on the way to slipping out of Russia's sphere of influence. As with Belarus, maintaining a pro-kremlin and corrupt state apparatus in Ukraine is seen by Putin as crucial to Russian interests.
Ukraine first applied to join NATO in the early 90s - way before they were anywhere near to being ready for that. It has been an on/off question for successive governments since. Without an initial move into the sphere of the EU though, the necessary changes would be so hindered by the endemic corruption typical of all states that still fall under Russia's influence as to make it a proposal with an indefinite time-frame. If anything, the invasion will serve to accelerate Ukraine's application. Beyond that, though, Ukraine joining NATO - unlike Romania, Lithuania etc., is as much an emotional issue for the Kremlin as a geopolitical one. In Putin's re-interpretations of history, Ukraine doesn't really exist.
NATO expansion is a convenient excuse for Putin, and a simplistic culpa nuestra for Kremlin apologists to wave around. More of a red herring if you really want to understand Putin's motivations. There's no specific reason why it would pose any more threat than the other nation on Russia's border already in NATO, other than the Polish corridor, but without full control over the Ukrainian military NATO or not would be a moot point in any case. More importantly, an actual written agreement to deny Ukraine the possibility of ever joining NATO would gift Putin an enormous moral and ideological victory. The very idea of the west as a bloc is based on the freedom of nations to choose their associations. Ceding that to pacify Putin would be little different to appeasing Hitler over Czechoslovakia and Poland.
A more crucial geopolitical issue is access to the Black Sea. Losing Crimea ('losing' as Putin sees it, there were hardly any ethnic Russians in Crimea prior to annexation under Catherine the Great, so the notion of it being historically Russian land is absurd) has long been a thorn in Putin's side. Claiming the whole land corridor, up to and eventually including Moldova, has been a secondary goal since the war began. Other than the warped sense that Ukraine is 'naturally' part of Russia, the primary driver of the war lies in the military and economic benefits of having ocean access in the South, a major issue for a country whose main access to sea routes is in the frozen North and far East.