The students of rabbi Shimon were studying and discussing the verse in Deuteronomy 2:9
“וַיֹּאמֶר יְהוָה אֵלַי, אַל-תָּצַר אֶת-מוֹאָב, וְאַל-תִּתְגָּר בָּם, מִלְחָמָה: כִּי לֹא-אֶתֵּן לְךָ מֵאַרְצוֹ, יְרֻשָּׁה–כִּי לִבְנֵי-לוֹט, נָתַתִּי אֶת-עָר יְרֻשָּׁה”
“”Then YHVH said to me, ‘Do not harass Moab, nor provoke them to war, for I will not give you any of their land as a possession, because I have given Ar to the sons of Lot as a possession.”
They explained that Ruth will be born to Moab, convert and from her the comes Mashiach. Because of that Moab received the protection of God.
“נְקֹם, נִקְמַת בְּנֵי יִשְׂרָאֵל, מֵאֵת, הַמִּדְיָנִים; אַחַר, תֵּאָסֵף אֶל-עַמֶּיךָ”
“Take full vengeance for the sons of Israel on the Midianites; afterward you will be gathered to your people.””
The students said that Jethro and Tziporah, Moses’s wife, came from Midian and why weren’t they protected. Where was Moses’s protection? It looks like they have greater merits than Moab to be protected.
Rabbi Shimon explained that Ruth didn’t yet come out of Moab so they needed protection. Jethro and Tziporah already left the Midianites and they didn’t need protection and the Israelites could take revenge from them.