What is the historical background of the current Ukraine crisis?

Question: What is the historical background of the current Ukraine crisis?

Short Answer:
The historic background of the current Ukrainian Crisis has more to do with Ukraine's division/independence from Russia (August 24, 1991) and the subsequent break up of the Soviet Union(December 26, 1991) than it does with any internal Ukrainian motivation. The "Ukrainian Crisis" is not a reflection of Ukrainian division. It's more rightfully named a Ukrainian Russian War. A war which pits the "divided" Ukranian people on one side and an only semi clandestine Russian force on the other.

Detailed Answer:
This is a very controversial question. Ukraine is indeed split between east and west along religious grounds Orthodox and Catholic. But the "split" concerning Russia does not follow this divide. 2/3rds of Ukranians self identify as Orthodox and only 10% Roman Catholic. Many Ukranians call themselves Christians and do not specify a branch.

CIA Factbook Ukraine
Ukraine's population is overwhelmingly Christian; the vast majority - up to two thirds - identify themselves as Orthodox, but many do not specify a particular branch; the OCU and the UOC-MP each represent less than a quarter of the country's population, the Ukrainian Greek Catholic Church accounts for 8-10%, and the UAOC accounts for 1-2%; Muslim and Jewish adherents each compose less than 1% of the total population

If Orthodox Christians actually favored reunification with Russia their would be no need for a Russian army to invade and occupy Eastern parts of the country.

Russo-Ukrainian War: Putin must be held accountable
Putin’s ex-advisor Vladislav Surkov, the architect of the Donbas war according to his former US counterpart in negotiations Kurt Volker, stated in February 2020, “forceful coercion to establish brotherly relations is the only method that has historically proven effective in relation to Ukraine.”

From everything we know in the west. The current Ukrainian crisis does not involve Ukranians beyond them being the victims of Russian aggression.

Russia Is Quick to Bend Truth About Ukraine New York Times
It is an extraordinary propaganda campaign that political analysts say reflects a new brazenness on the part of Russian officials. And in recent days, it has largely succeeded — at least for Russia’s domestic audience — in painting a picture of chaos and danger in eastern Ukraine, although it was pro-Russian forces themselves who created it by seizing public buildings and setting up roadblocks.
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In essence, Moscow’s state-controlled news media outlets are loudly and incessantly calling on Ukraine and the international community to calm a situation that Ukraine, the United States and the European Union say the Kremlin is doing its best to destabilize.

It is true that there were numerous historical propaganda justifications for the "split", also for the "intervention". They are pretty much all bogus. They are just political cover for the Russian land grab. A war instigated by Russia, manned by Russian citizens both active duty and paramilitary forces and paid for Russia. Clearly much of the Western world believes Russia responsible because the sanctions Russia has been facing now since their semi clandestine invasion.

What we know is Russia sponsored paramilitary forces which caused civil unrest in the Ukraine which broadened into a sudo clandestine invasion of Ukraine by the Russian military.

A Separatist Militia in Ukraine With Russian Fighters Holds a Key
When more than 40 of the militia’s fighters were killed in heavy fighting near the Donetsk airport last month, separatist leaders said that 33 were Russian citizens. Their bodies were sent in a refrigerated truck to the Russian border for repatriation.

• Rebels without a Cause: Russia’s Proxies in Eastern Ukraine

Pro-Russia Troops Take Symbol of Ukraine Uprising
Increasingly, a cadre of commanders with Russian citizenship like Mr. Borodai and a shadowy military commander named Igor Strelkov seem to be seizing control of the often rudderless rebellion as clashes with the Ukrainian Army intensify.

We know the "little green men" wearing Russian Uniforms, driving Russian tanks, and carrying Russian weapons; were in fact Russian active duty military personnel because they posted photos and notices on social media proclaiming such.

• Without Realizing It, Russian Soldiers Are Proving Vladimir Putin Is Lying About Eastern Ukraine
• Russian military bans selfies to prevent soldiers revealing locations

Russia Has Deployed Thousands of Tanks, Troops to Ukraine, Top Official Says
Moscow sends a ‘continuous flow of munitions’ into the conflict zone as part of a broader scheme to degrade Ukraine’s army.

• European court of human rights (ECHR) evidence of Russia responsibility viewable online
• New evidence emerges of Russian role in Ukraine conflict

It is after all why Russia is currently under sanctions by many countries including US, EU, Australia, and Canada. We know Russian paramilitary groups were enlisted to enter Ukraine and destabilize the country because the leaders of these groups are mostly Russian and the men involved in their militia's supporting separation with Ukraine are also mostly Russian. That's over an above the actual Russian armies involvement there.

Now as to your question about historical background. When the Ukraine separated from Russia, it took with it Russia's best black sea ports. These ports are an important asset for Russia and something down through time under the Czar's something Russia historically was willing to go to war over. Russia want's back what it once had. It's as simple as that.

One could look to the 1596 Union of Brest when the Ukrainian bishops (in Poland-Lithuania) chose not to recognize the new Muscovite Patriarch and formed the Uniate communion (Greek Catholic Church of the Slavic Rite). The Uniates formed in what is today western Ukraine and Belarus, whereas those across the border in the expanding Muscovy (what we now call Russia) chose to become Russian Orthodox.

One could make a case that this religious division kept the border between Poland-Lithuania and Russia as a cultural fault line even after the political border shifted westward with Russian expansion.

You can see the remnants of the old border in other ways too. Western Ukraine was considered part of the Jewish Pale of Settlement in the nineteenth century whereas eastern Ukraine was not.

During the "Russian Civil War" of 1918-1921 (which Norman Davies claims is a misnomer), the short lived "West Ukrainian Republic" fought both Pilsudski's Polish forces and the Russian backed Bolshevik army. Unfortunately, the Soviet reconquest of Ukraine kept us from seeing how an independent West Ukraine would behave.

In "Clash of Civilizations", Samuel Huntington uses the old Polish-Lithuanian/Russian border as the dividing line between the Western and the Orthodox civilizations. Interestingly, he makes several mentions of the possibility of Ukraine splitting in two. This did not happen in the almost 20 years since he wrote it, but it remains a possibility.

Sources:

"Europe: A History" by Norman Davies (1996)

"The Clash of Civilizations and the Remaking of World Order" by Samuel Huntington (1996)