Apparently at the time of Ruth, relations were good enough that Elimelech and his family did not have problems living in Moab during a famine in Judah, and David sent his parents to Moab for safety during the years he was hiding out from Saul. We see Ruth accepted in Judah when she came back with Naomi, but she had accepted Naomi and Judah's God as her own. She is also depicted as a loyal and responsible daughter to Naomi, willing to work at a job kept for the poor.
There were continuing problems with Israelite men marrying outsiders, and allowing them to continue their worship of pagan gods. This was the downfall of Solomon, who even built sanctuaries for his political wives' gods. We see King Ahab marrying a aggressively pagan Phoenician princess, who not only kept her own gods, but tried to eliminate the prophets of God and the worship of Israel's God from the kingdom. Jezebel's daughter Athaliah, who had married King Jehoram of Judah, was her mother's daughter, and brought the worship of Baal to Jerusalem. After the "premature" deaths of her husband and son, she killed off all the other heirs to the throne of Judah, including all but one of her own grandchildren. The infant Joash was rescued and hidden by an aunt, who was married to the high priest. Six years later, the high priest brought out Joash for his coronation, and Athaliah was killed by the temple guards as she tried to escape.
When the Jews returned to rebuild Jerusalem after the Babylonian captivity, Nehemiah, the governor, and Ezra, the leading priest, were concerned about the number of foreign wives among the returnees. Looking back at the "bad examples", it was decreed that they all had to be divorced and deported; there is no record of any consideration given to the religion of the wives. The Book of Ruth was written and promoted during this period as an antidote to the wholesale discrimination against foreigners.