[Rune Son of Bjarne]

You might be right that Morgoth did not fear Melian and Thingol and that they where no real threat to him, but I am confident that he would eventually seek to destroy Doriath with military force if it had stood long enough.

As I understand it Morgoth did not only wager war against those that where a threat to him, he sought to defeat and enslave the whole world and even thought Doriath could not challenge him it was too great to be ignored.