"'In the visions I saw while lying on my bed, I looked, and there before me was a messenger, a holy one, coming down from heaven. He called in a loud voice: "Cut down the tree and trim off its branches; strip off its leaves and scatter its fruit. Let the animals flee from under it and the birds from its branches. But let the stump and its roots, bound with iron and bronze, remain in the ground, in the grass of the field."'" NIV translation
Now, we see the dream start to become a nightmare as it is decreed that the magnificent tree be cut down. Many times a tree is used as the symbol for a man in the Bible and Nebuchadnezzar must have thought of himself as this magnificent tree. It must have shocked him to here the angel say that it must be cut down. The good news is that they did not uproot the stump so there was the ability for the tree to grow again. We see that the stump was restrained from growth on its own by these bands. We, like Nebuchadnezzar, can become arrogant and see ourselves as a mighty tree but we must remember to worship the Creator of the tree and give Him the praise instead of becoming arrogant.
"'Let him be drenched with the dew of heaven, and let him live with the animals among the plants of the earth. Let his mind be changed from that of a man and let him be given the mind of an animal, till seven times pass by him.'" NIV translation
Now, we see that this man will be driven to act like an animal for seven years.
Nebuchadnezzar may have still held out hope that it was speaking of someone else but deep inside he must have been horrified at the thought that it was speaking about him.
"'"The decision is announced by messengers, the holy ones declare the verdict, so that the living may know that the Most High is sovereign over the kingdoms of men and gives them to anyone he wishes and sets over them the lowliest of men."'" NIV translation
God explains the purpose of His actions which is so that men can understand that He is in charge of all creation. Part of that being in charge is the fact that He puts whoever He wants in positions of authority. Usually, these are not the most superior of men but they are there because of the will of God and for His purposes. This is true today as well in that God puts our political leaders in their place to fulfill His purpose and many times they are humbled by the fact that they are being used and cannot do it on their own.