Who was the Navi?
Radak: It was the Navi mentioned above (verse 13).
What was the command "Hischazak"?
Radak #1: Prepare yourself for war, for [Melech Aram] will return at the end of the year. Malbim - miracles do not always occur. Prepare [strategy and] Gevurah for war, i.e. [foot] soldiers and horsemen.
Radak #2: Strengthen yourself with good deeds and uproot idolatry, for you saw that Hashem is Elokim, and "Sheker ha'Sus li'Sshu'ah" (Tehilim 33:17). Hashem gave this gigantic assembly into your hands!
Why did he say "v'Da u'Re'e Es Asher Ta'aseh"?
Malbim: Prepare strategy for war. (Refer to 20:22:2:1.)
What is "li'Shuvas ha'Shanah"?
Rashi citing Targum Yonasan: It is the time of the end of the year. Radak - it is when the sun is Shav (returns) to the same place it was at the beginning of the year.
