Jeremiah 27

And now I have given all these lands into the hand of Nebuchadnezzar the king of Babylon, My servant; and the beasts of the field I have also given him to serve him. v.6

Read chapter 27
Isaiah walked naked, Ezekiel had to lay on his side for 430 day, Jeremiah had to carry bonds and yokes on his neck (v.2).  These all were visual signs through the prophets to Israel and Judah of captivity that was coming on them.  This wasn't just signs of "beware of evil," but of sure punishment that was to come on them for their sins turning away from the Lord.  These signs were to speak to the people saying that punishment was sure to come and that they ought to submit under this authority that they may live than be destroyed in the land. 
God raises up a gentile king- Nebuchadnezzar, King of Babylon- who will destroy Judah, the southern kingdom, by murdering, destroying their land, carrying away their plunder and taking many away captive.  God calls Nebuchadnezzar "My servant."  How can a wicked gentile king be God's servant?  Well for one Nebuchadnezzar didn't think he was anyone's servant not realize God was using him.  He was used as God's instrument to carry out His will- whether Nebu knew it or not.  Also, God is the creator and proprietor of the whole earth and can give it over to whomever He pleases and so He turned it over to Babylon for a time.
How have people that are not believers in the One True God been a sign to you of how God wants you to act according to His will?