Illarionov demonstrates point by point that Russia had been preparing for a major war up to April 26, but messages coming from Moscow from April 28 onward have had an markedly different flavor. What happened in the 48 hours between the evening of the 26th and the 28th?
A partial answer can be found in press releases from the Russian and American Defense Ministries published on April 28. The press releases speak of an hour-long conversation between Russian Defense Minister Sergey Shoygu and US Secretary of Defense Chuck Hagel. I'll translate Illarionov's summary and conclusion (points 19-21).
19. A brief summary of the documents cited above:
"In view of the Russian army concentration on the Russian-Ukrainian border and the presence of clear signs of its readiness to invade continental Ukraine at any moment, the U.S. administration temporarily stopped trying to persuade its Moscow counterpart, as it had been doing the previous two months, and began speaking a language the Kremlin could understand. In the language of the Russian Defense Ministry's press release the signal is described as follows: "The Russian Minister described the increased activity of US and NATO armed forces at Russian borders in Eastern Europe as unprecedented." In the language of the US Defense Ministry the signal that was sent is described in this way: "Hagel emphasized to his Russian counterpart how dangerous the situation in Ukraine remains and that Shoygu "reiterated his assurance that Russian forces would not invade." Sec. Hagel also repeated his call for an end to Russia's destabilizing influence inside Ukraine and warned that continued aggression would further isolate Russia and result in more diplomatic and economic pressure."
Thus, it can be stated that the unprecedented increased activity of US and NATO armed forces at Russian borders in Eastern Europe, together with diplomatic and economic pressure averted the invasion of Ukraine by Russian army forces and halted the continually increasing momentum of a new war in Europe.
20. In other words, on April 26-28, 2014, the US administration and armed forces began to take measures apparently similar to those taken by the US administration and armed forces on August 11-12, 2008, which halted the Russian army offensive towards Tbilisi and led to a cease-fire in the Russian-Georgian war.
21. If the US administration and armed forces together with the government of Ukraine had begun to take measures not on April 26-28, 2014, but exactly two months earlier, on February 26-28, then the Russian-Ukrainian war would have been halted at its very earliest stages, Ukrainian Crimea would not have been occupied and illegally annexed by Russia, a multitude of documents of international law would not have been violated, the system of international relations and global security which had been taken seven decades since WWII to create through the efforts of hundreds of nations at the cost of hundreds of billions of dollars would not have been destroyed, and dozens, if not hundreds, of people would not have lost their lives.
What remains to be seen now is whether the de-escalation is just a tactical retreat or a long-term change of course.