How will Russia's war with Ukraine end? Here are 5 possible outcomes

Less than two weeks into Russia's invasion of Ukraine and the country's people and armed forces continue to mount a staunch — and undeniably brave — resistance against Russian forces.

But for all of Ukraine's heart and courage in facing down multiple, sustained attacks from Russia's military in the north, east and south of the country, many analysts and strategists believe it is only a matter of time before Ukraine is overwhelmed by Moscow's military might.

What comes next for Ukraine could be bleak, these experts say, with many expecting a long and drawn-out conflict, noting that even in the most positive scenario — that Russia withdraws its troops and Ukraine remains a sovereign nation — Europe is unlikely to return to the prewar status quo.

CNBC takes a look at the possible outcomes for Ukraine and what might happen in each of them:

Close watchers of the Russia-Ukraine war say the fluid and rapidly changing nature of the conflict makes it hard to gauge what will happen next in Ukraine, with both Moscow's and the West's next moves unpredictable.

However it's widely expected that Russian President Vladimir Putin, loathing Ukraine's current pro-Western government and its aspirations to join the EU and NATO, wants to install a pro-Russian regime in Kyiv.

Just how and when (and if) that happens is uncertain but Eurasia Group's base-case scenario for the next three months is for Russia to gain "patchy control of eastern Ukraine, up to the Dnipro River" and to capture the capital Kyiv after a protracted siege, and for "a Russian-backed puppet government" to be established.

Eurasia Group's chairman, Cliff Kupchan, and colleagues added in a note Thursday that "a rump Ukrainian state" is likely to be led from Lviv, a city in Ukraine's west and near the border with Poland, with the semi-exiled government likely to receive "heavy western support."

The analysts predicted refugee flows of 5 million to 10 million people from Ukraine to Western Europe.

In such a scenario, Eurasia Group predicted that NATO, which has so far refused to intervene militarily in the conflict (Ukraine is not a member of the military alliance), would provide "significant military assistance to the western Ukrainian state and materiel [military materials and equipment] to support insurgency in eastern Ukraine." But they added that this could lead to the risk of airborne clashes between Russian and NATO aircraft.

Russia's military strategy has at times been beset with logistical problems, confusing the picture of what Russia's main or immediate goals are.

To date, only one city has definitively fallen to the Russians since the invasion began in the early morning of Feb. 24 — Kherson — although others like Mariupol, in the south, appear to be perilously close amid food, water and power shortages.

Resistance to Russian forces is likely to get tougher as the war progresses and Russia pulls out the stops to seize more territory.

Scott Boston, a senior defense analyst at the Rand Corp., told CNBC on Friday that the Russians "have a whole lot of combat power left and a lot of capacity to scale up the violence, which seems to already be happening. This thing could really drag on for a long time."

Some analysts agree that any patchy control over Ukraine by Russia could lead to some kind of partitioning of the country, particularly as Russia becomes firmly entrenched in eastern Ukraine — particularly in the Donbas region where it recognized the independence of two pro-Russian republics ahead of its invasion of the wider country.

Taras Kuzio, a research fellow at the Henry Jackson Society, wrote in an article for the Atlantic Council on Thursday that Moscow has indicated that it is aiming at "the complete military conquest of Ukraine followed by a partition and a massive purge of the civilian population."

"Putin's apparent objective is to eradicate all vestiges of Ukrainian identity while condemning the country to a grim future as a military dictatorship locked firmly inside a new Russian Empire. This nightmarish vision tallies closely with Putin's own stated objectives for the current military campaign along with his long record of public contempt and animosity towards Ukrainian statehood," he said.

There are many questions over who could lead a loyalist regime in Ukraine, one that could resemble that of Belarus' Alexander Lukashenko. Kuzio noted that there has been speculation about Moscow seeking to install former Ukrainian President Viktor Yanukovych, who was stripped of his powers by Ukrainian lawmakers during the 2014 Maidan Revolution and fled Kyiv for Russia.

"This would be entirely in keeping with Kremlin propaganda, which has insisted for the past eight years that Yanukovych was illegally removed by a Western-backed coup," Kuzio noted.

Most warn that Ukrainians would continue to fight against any puppet regime, with the conflict descending into an insurgency with those Ukrainians left in the country attempting to topple any such regime by any means available.

Close watchers of Russia like Tim Ash, an emerging markets strategist at BlueBay Asset Management, have said Russia is likely to face a long, drawn-out, pricey and painful occupation of Ukraine.

"Assuming Putin wins the military war the trillion dollar question is how he wins the peace in Ukraine ... Ukrainians have had 30 years of freedom, which they relish, and how can Putin turn the clock back to 91' [the collapse of the Soviet Union] without brutal suppression which would further make him, and his puppet regime in Kyiv, international pariahs. This is not 1945, 1956 or 1968 where Soviet troops/the NKVD [the Soviet law enforcement agency] did bludgeon civilians into submission, but 2022," Ash said in emailed comments on Feb. 25, a day after Russia invaded Ukraine.

"Ukrainians will resist long and hard even if the formal military battles end. And news 24/7 and the internet will expose Putin's brutality for all to see."

There's of course the possibility that a Ukrainian fightback doesn't pose a significant challenge to Russian forces that remain in Ukraine — after all, thousands of fighters are civilians who taken up arms and have been hastily trained.

Other analysts warn of a "quagmire" — where there is no easy solution for what would likely be a heavily destroyed Ukraine, or for Russia — if an insurgency continued long term.

In this scenario, strategists at the Atlantic Council's Scowcroft Center for Strategy and Security program, Barry Pavel, Peter Engelke and Jeffrey Cimmino, noted that Russia's victory in Ukraine would be a "pyrrhic one," that is, a victory not worth winning because so much is lost to achieve it.

In this scenario, the strategists noted that a Ukrainian insurgency could force "a significant, sustained human and financial toll on Russia" as it would be required to devote far more of its resources over a much longer period of time than it had anticipated. In the meantime, NATO countries "would likely provide covert but very robust defensive assistance to the Ukrainian resistance."

In this scenario, "the conflict drains Moscow's coffers and resolve, ultimately forcing a withdrawal after much violence and death," an outcome that has echoes of Russia's ill-fated, unpopular and costly invasion of Afghanistan in 1979, a conflict that lasted 10 years and led to the deaths of 15,000 Russian soldiers.

In this scenario, the strategists noted, Russia would realize it has "once again fought an unwinnable war, the proverbial quagmire that has trapped many powerful states through history."

While this scenario might appear positive for Ukraine, with Russia becoming a pariah state at a global level and withdrawing after a costly invasion, Ukraine would be "devastated" in the process, the strategists said.

The Western military alliance NATO has repeatedly refused to directly intervene in the Russia-Ukraine war as doing so would likely bring it into direct conflict with Moscow which, for its part, has warned that any country that "interferes" in what it calls its "special military operation" in Ukraine will face untold consequences.

Countries on the EU's (and NATO's) eastern flank like Poland, Romania and the Baltic states, all of which have seen their NATO deployments bolstered in recent weeks, are extremely nervous about the potential for the conflict to spill over into their own territories.

If Russia prevails in Ukraine, analysts including Ash have warned of a new "Iron Curtain" descending on Eastern Europe, creating two opposing geopolitical blocs reminiscent of those in the Cold War — the EU (and NATO nations) on one side of a potentially militarized border and Ukraine and other countries in Russia's political orbit (such as Belarus and Moldova) on the other.

Such a situation is a tinderbox in Europe, Ian Bremmer, Eurasia Group's president, said in emailed comments Monday. He noted that it's a "non-starter" for the West to send troops to fight alongside Ukrainians or to implement a no-fly zone over Ukraine "because that leads to direct confrontation between NATO and Russian troops and accordingly risks World War III."

"Anything short of that is fair game: you can send fighter jets and other advanced weapons systems to the Ukrainians, provide Ukraine with real time intelligence on the disposition of Russian forces, and take economic measures without limitation to destroy the Russian economy," he said.

But Bremmer believes that Putin still perceives this kind of help "as acts of war taken by the United States and NATO allies against Russia, meriting retaliation."

Bremmer said Russia may therefore resort to more indirect attacks including cyberattacks against critical infrastructure, disinformation campaigns, and even the possible sanctioning of terrorism in and against NATO countries.

"It remains highly unlikely Russia would launch direct military attacks against NATO forces, given that's understood by NATO to be a tripwire for a broader war ... but support for Chechen terrorist attacks into frontline NATO states delivering all these weapons? That's another matter. NATO would be unlikely to respond directly with military strikes against a nuclear power; the only way to prepare is greater intelligence efforts to prevent or at least blunt the effectiveness of the efforts," Bremmer said.

Strategists based in Eastern Europe are under no illusion as to whether NATO could get dragged into the conflict.

Michal Baranowski, director of the German Marshall Fund's Warsaw office, told CNBC just hours into Russia's invasion that Putin "told us what he wants to do, he wants to change the government in Kyiv and when he was laying out his demands he was talking about the eastern flank of NATO and the rest of Europe as well. So buckle up, we need to be deterring not only attacks on Kyiv but the rest of the lines."

"The world has changed. There is no going back … we are in an entirely new era," he said.

"We are in for a very long fight, this is not going to be short, this is not only going to be about Ukraine. ... This is probably the biggest challenge that we are seeing in Europe since World War II," he said.

Analysts of course agree that an unequivocal withdrawal of Russian armed forces from Ukraine would be best possible outcome for the country in its dire situation.

Analysts at the Scowcroft Center noted that, in their "rosiest" possible scenario for how the Ukraine conflict could end, Ukraine could see its own defensive capabilities bolstered by NATO, allowing its military and civilian resistance to "overcome the odds and grind Moscow's advance to a halt."

In this hypothetical scenario, Putin would be prevented from toppling Kyiv's government and establishing a puppet regime, while "the determination and skill of the Ukrainian resistance forces a stalemate on the battlefield that favors the defenders," the Atlantic Council's strategists Pavel, Engelke and Cimmino noted.

Indeed, in this "miracle" scenario, the analysts said that the Kremlin realizes that Russia "will pay an exorbitant price" for its invasion of Ukraine and, facing the prospect of a long and costly slog in Ukraine, coupled with economic collapse and diplomatic isolation, Putin would order a withdrawal of his troops.

Still, even this outcome where Ukraine remains a sovereign democracy and NATO is faced with an improved security situation could be "fraught with danger," the analysts warned.

"The short war has claimed thousands of lives on both sides, leaving widespread bitterness in its wake. And although a democratic Ukraine emerges intact if not unscathed, its still-dangerous neighbor faces an uncertain future with the Russian political landscape at a tipping point. Whether the country leans toward greater authoritarianism under Putin, or away from him altogether, will largely determine how Russia behaves with the rest of the world," they added.