Ezra 8, 9, & 10

Ezra 8
Ezra gathered all those to travel with him to Israel thanks to Artaxerxes' decree. Once gathered, Ezra realizes that there are no Levites to make the trip! Ezra seemed to know people in high places, so he was able to gather together a few Levites after making a request.

Ezra then proclaimed a fast to humble themselves before the Lord and ask for safe travel. Ezra says he did this because he could not ask the King for protection from the enemy on the way due Ezra's boasting in the Lord. The Lord heard Ezra's entreaty and gave them safe passage.

Ezra and those with him made offerings to the Lord. They left for Jerusalem and arrived safely. Those who traveled offered burnt offering in Jerusalem.

Ezra 9
Princes approached Ezra to tell him of the unfaithfulness of the Israelites in marrying themselves to other nations. Ezra was clearly distraught by this notion so much that he tore his clothes and didn't change them until evening. He just sat down, appalled. I'm not really sure what this is about because I have looked back and I can't find any precedent where God says not to marry outside of Israel. Can anyone help?

Ezra continues and humbles himself before the Lord, condemning these marriages. He admits that none can stand before God because of their iniquities in verse 15.

Ezra 10
Ezra gathers the remnant and confesses before them, charging them with the task of 'putting away' these who have joined with those outside of Israel. Ezra goes through this investigation of men concerning their marriages, and then, he lists them.

I get the concept that marriage to an unbeliever is not desirable (2 Corinthians 6:14), and I guess I can see that principle in this story; but I have no old testament reference to the fact that the Israelites were told not to do this.

Lord, I pray for understanding and guidance in the truth.