Surely if Putin was determined on playing the part of a conqueror intent on restoring the old political order he’d be a bit more aggressive and conduct the war on a larger scale. After all, the last time Russia reconquered Ukraine they did it in only a couple of years, and that was in the midst of revolution.
Putin is very cautious, and I would say - fearful. He took a step and watched how the West reacts to it. When its reaction seemed weak or uncertain to Putin, he took the next step.
The West has strong levers of influence on Putin and his elite, and the Kremlin is well aware of this. And when Putin introduced regular troops to Ukraine (end of August 2014) - the States held an extremely unpleasant sanction for Russia, although they never voiced it publicly. They collapsed the price of oil, having previously agreed it with Saudi Arabia. This short visit of Obama took place at the end of March 2014, immediately after the annexation of Crimea by Russia.
A new round of Russian aggression in Ukraine brought this non-announced agreement into action. The price of oil has plummeted from $ 120 per barrel to $ 45. Since the tax on hydrocarbon exports produced more than half of the Russian budget’s revenues, it dealt a heavy blow to Russian finances and the economy. As I said, Russia's GDP fell from 2.3 trillion dollars (2013) to 1.3 trillion (2015). And this forced the Kremlin to sharply reduce its appetites in Ukraine and forced it to abandon the escalation of aggression against this country.
The second factor was the rapid growth of Ukraine’s resistance to Russian aggression. The Kremlin thought that the Ukrainian troops were not capable of anything, but this was not the case. And Girkin (who headed the "army of DNR in summer 2014) when he returned to Moscow from the Donbass admitted that
the Ukrainians are fighting well.
The third factor was that after Russia's annexation of the Crimea and the invasion of the Donbass Russia's popularity in eastern Ukraine began to decline rapidly. And among the Ukrainians who went to the front as volunteers the number of those who lived in the east of Ukraine sharply increased. Among the soldiers who are fighting against the Russian forces in the East there are also quite a lot even those who have moved from the occupied territories of Donbass. This was told to me by my nephew that was fighting as an artillery officer in the Ukrainian troops in the Donbas. Under his command there is even a string of former Berkut soldiers who at the beginning of 2014 opposed the Protestants on the Maidan.
Another factor was that after the collapse in oil prices Russia began to become noticeably poorer. And the era of its rapid prosperity was over. It has ceased to be attractive to the majority of those Ukrainians who until 2014 treated Russia with great sympathy.
I often write on the Donetsk city forum -
Донецк Форум. Донецкий форум. - as VoxPopuli.
And I can say that compared to the period until 2014 most of its participants from Donbass began to criticize Russia and its invasion of this region - and support Ukraine
In addition, the Kremlin knew how strong the resistance of Ukrainians could become in the format of a partisan war, if it carries out a massive invasion of Ukraine. Even a small part of Ukrainians in the format of partisans of UPA - Ukrainian Insurgent Army - had a strong resistance to the Russians from 1944 to the early 1950s with a solid support of local peasants. And the Kremlin needed extraordinary punitive measures (including the deportation of 300 thousand of the western Ukrainians for supporting the partisans) diluted with two amnesties for the partisans to suppress this resistance
The annexation of the Crimea in March 2014 was largely due to Putin’s desire to improve his rating, which by then had fallen dramatically. A high rating among Russians is extremely important both for Putin and for the Kremlin in order to maintain stability in that country. The annexation of the Crimea made this rating a record. But it caused big problems for Russia, lowered the standard of living of Russians and put an end to the "
Putin miracle."
In May 2014, 83% of Russians polled expressed support for the president, while the number of those who are not satisfied was down from 34% to 13%. The euphoria from the capture of the Crimea did not last too long. And now Putin’s rating (VCIOM poll) fell to 32%