וַיֹּ֗אמֶר הֲיָדַ֙עְתָּ֙ לָמָּה־בָּ֣אתִי אֵלֶ֔יךָ וְעַתָּ֣ה אָשׁ֔וּב לְהִלָּחֵ֖ם עִם־שַׂ֣ר פָּרָ֑ס וַאֲנִ֣י יוֹצֵ֔א וְהִנֵּ֥ה שַׂר־יָוָ֖ן בָּֽא׃
Then said he: Knowest thou wherefore I come unto thee? and now will I return to fight with the prince of Persia; and when I go forth, lo, the prince of Greece shall come.
Read halakhic rulings (Jewish law) on Daniel 10:20: and classical halakhic sources.