Russia had to intervene directly (even if it denies it) to avoid defeat in eastern Ukraine.
Let's just note from the article a violation of the rules of war by those rebels that is casually described:
Major [Note: what a rebel commander called himself] took two boxes' worth of unexploded ammunition from the wrecked fighting vehicles to downtown Komsomolske, where burly local men took them into an apartment building near the abandoned fighting vehicle turned playground. A few minutes later, a group of scruffy rebel fighters pulled up in an appropriated ambulance, loaded the ammunition into it, and drove away.
Using an ambulance to carry ammunition is a violation of the rules of war because it puts all ambulances at risk of enemy attack. If ambulances are used for the sick, injured, and wounded, it is safe to hold fire. When you know the enemy will use them as supply vehicles (or personnel carriers) all ambulances become targets.
And in that case the responsibility for the deaths lies not with the side that fired on the ambulance but with the side that started using ambulances as military vehicles.
But back to the point, Ukraine needs to save their army in the east. I doubt if more than elements of four brigades are in combat there--plus National Guard light infantry and special forces--but since Ukraine's military is pretty small, Ukraine can't afford to lose that force in combat with the Russians.
As I wrote recently, if the Russians invade Ukraine--and Russia has done that openly now with maneuver units--Ukraine needs to do three things:
If Putin does escalate to openly waged warfare against Ukraine to take eastern Ukraine, Ukraine needs to do three things: preserve the Ukrainian army; wage irregular warfare in eastern Ukraine to stress Russia's still-inadequate ground forces; and strike Sevastopol.
There is no shame in retreating (in good order, hopefully, but just escaping is valuable--ask the British after Dunkirk) before a suddenly stronger enemy if the alternative is dying in place fighting a hopeless battle.
The existence of Ukraine's ground forces is an important objective. Save it and Ukraine can resist invasion and support efforts to reclaim lost ground. Lose it and it is up to partisans to resist Russian occupation.
Even a ceasefire might be okay if it is the only way to allow Ukraine to save the army and prepare for further war rather than just pave the way for Russia to cement control over eastern Ukraine.
The second has to be the means to fight for eastern Ukraine if the regular military is too weak to face Russian regulars.
The last has to be done to deny Russia the luxury of being the side that gets to define the war's scope to their benefit.
UPDATE: Breaking news is that Ukraine and "the rebels" have agreed to a ceasefire. Russia continues to deny the obvious that they are the major combatant on the other side.
UPDATE: Ukraine's president confirms the ceasefire. Is this a Russian victory or a pause to regroup?
I fear we have peace for our time.