Based on an article by Rabbi A Moss
Why doesn't Judaism proselytize?
Jim, do you have brothers and sisters? If you do, I'm sure you'll agree, that each of you is different. One of you may be musical, another a sports freak, and yet another more academically inclined. Each of you is an individual, and that should be encouraged.
It would be wrong of parents to treat all their kids the same. If they would have forced you all to play violin, or soccer, or to read Shakespeare, it would have been counter-productive.
Good parents know, that each child has to be allowed to develop in their own unique way.
The nations of the world, are all G-d's children. He doesn't treat them all the same, because they aren't all the same. He wants each nation to develop in its own way. So each nation has a different path to reach their full potential.
To the Jewish nation, he gave the Torah as our way of expressing our souls. But Judaism is not for everyone. We don't believe that a non-Jew needs to become Jewish to find G-d, just like a musically-talented child shouldn't be forced to play soccer. Anyone is welcome to convert, but that is up to them.
G-d created different nations, because each has a unique contribution to offer the world. Maybe your "mission" should be, to ensure that your nation fulfils that purpose.