In the letter Putin directly states that he considers significant amount of Ukrainian territory as not-really-ukrainian, but a "present" from Bolsheviks or something. He also states that all of these territories now are "subject to discussion". I think it should be quite obvious to anyone (including the author) that Ukraine (like any other country) wouldn't be inclined to willingly discuss giving away its lands, so conquest is the only logical consequence.
Relevant excerpts:
You want to establish a state of your own: you are welcome! But what are the terms? I will recall the assessment given by one of the most prominent political figures of new Russia, first mayor of Saint Petersburg Anatoly Sobchak. As a legal expert who believed that every decision must be legitimate, in 1992, he shared the following opinion: the republics that were founders of the Union, having denounced the 1922 Union Treaty, must return to the boundaries they had had before joining the Soviet Union. All other territorial acquisitions are subject to discussion, negotiations, given that the ground has been revoked.
In other words, when you leave, take what you brought with you. This logic is hard to refute. I will just say that the Bolsheviks had embarked on reshaping boundaries even before the Soviet Union, manipulating with territories to their liking, in disregard of people's views.
...
In essence, Ukraine's ruling circles decided to justify their country's independence through the denial of its past, however, except for border issues. They began to mythologize and rewrite history, edit out everything that united us, and refer to the period when Ukraine was part of the Russian Empire and the Soviet Union as an occupation. ...