"Look! He advances like the clouds, his chariots come like a whirlwind, his horses are swifter than eagles. Woe to us! We are ruined!" NIV translation
God describes the coming judgment on Judah and Jerusalem at the hands of the Babylonians. They were known for having a fast moving army and being unstoppable.
"Jerusalem, wash the evil from your heart and be saved. How long will you harbor wicked thoughts? A voice is announcing from Dan, proclaiming disaster from the hills of Ephraim." NIV translation
Even in the face of this coming judgment, God is calling the people to repent. He urges them to remember what has happened to the northern kingdom.
"'Tell this to the nations, proclaim concerning Jerusalem: "A besieging army is coming from a distant land, raising a war cry against the cities of Judah. They surround her like men guarding a field, because she has rebelled against me,"' declares the Lord." NIV translation
Why would God judge His people? We see the answer here as they are told to tell the nations of this judgment. Israel and Judah were chosen to be God's people and an example to the pagan world. They had turned away from their purpose and so God was going to use this judgment as an example to the nations of the penalty for rejecting the One True God.
"'Your own conduct and actions have brought this on you. This is your punishment. How bitter it is! How it pierces to the heart!'" NIV translation
The people are reminded that this judgment is a direct result of the choices that they have made. As Creator of the world, God has the authority to say what is right and what is wrong. He gives men the freedom and the choice to follow or to go their own way. With this choice comes responsibility and consequences.
"Oh, my anguish, my anguish! I writhe in pain. Oh, the agony of my heart! My heart pounds within me, I cannot keep silent. For I have heard the battle cry." NIV translation
The prophet now shares how bringing this bad news is affecting him. Nobody wants to be the bearer of bad news and this is extremely bad news. Jeremiah loves the people and he loves them enough to tell them the truth even though they did not want to hear it and it is breaking his heart.