The Ukraine used to be part of the Soviet Union and was basically controlled by Russia, as was all the Soviet Union. When the Soviet Union broke up, the Ukraine became independent of Russia.
Now, the majority of the people of the Ukraine, but not ethnic Russians living in the Ukraine, want to remain independent of Russia and would prefer closer economic and political ties with Europe and the west rather than with Russia. Germany and Europe also would like the Ukraine to not be dominated by Russia.
There has been turmoil in the Ukraine recently, and Russia wants to take advantage of the opportunity to gain greater control of the Ukraine, threatening and using military force to achieve their aims. Russia may have helped stir up the turmoil. The United States has warned Russia to stay out of the Ukraine.
But President Obama's credibility here is weak. I think it is unlikely he will back up his warnings with force, and President Putin of Russia probably knows this.
If Russia uses military force or the threat of military force to gain dominance over the Ukraine, and the United States, through inaction and weakness, lets it happen, this will put more doubts in the minds of German leaders and the people of Europe that the United States is not trustworthy as an ally. And if they cannot trust the United States to protect Europe from Russia, then they must increase their own military strength to be able to protect themselves.
In effect, this can be a wake-up call, or part of a wake-up call, for Europe to develop a much greater military strength, and unity, than it has now.
For if the United States lets Russia take the Ukraine, what about other countries in Europe that used to be part of the Soviet Union but now are independent? And what about those countries that used to be part of the communist block and were controlled by the Soviet Union, like Poland and Hungary? And if the United States allows Russia to gain dominance over those nations, by force or the threat of force, how can Germany itself feel safe?
Germany has the technical and industrial might to become a nuclear power whenever it wants. France and the U.K. are already nuclear powers. Germany may decide, at some point, to become a nuclear power to have its own independent deterent against Russian agression, and it may decide also to vastly expand its army and airforce. It has the money to do it.
Then, at a later time, such military forces can be used, not just against Russia, but against other powers as well, even the United States.
American weakness in the face of Russian agression can be the motive for the beast power to rise and become strong in Europe and for Europe to feel the need to be more independent of the United States. Military power in Europe can be developed first for self-defence, but later used for aggression.