Why will Hashem call to Shamayim?
Rashi: It is to judge the Sarim of the nations above.
Radak: He calls to the angels to judge Yisrael's case against the nations - "Shamah Hanchas Hashem Giborecha" (Yo'el 4:11), "va'Yetzei Mal'ach Hashem va'Yach b'Machaneh Ashur" (Melachim II, 19:35).
Radak citing Sanhedrin 91b: Shamayim refers to Neshamah, and "ha'Aretz" to the body. On the day of judgment there will be Techiyas ha'Mesim, and the body will be judged with the Neshamah.
Malbim: He will summons Shamayim and the land to the case of His nation. All will join to investigate the case.
Why will Hashem call to the land?
Rashi: He will judge the kings of Egypt.
Radak: He calls to the people of the land. Each man will kill his brother; Hashem will bring frenzy among them - "Cherev Ish b'Achiv Tihyeh" (Yechezkel 38:21).
Refer to 50:4:1:3-4.
What is "Ladin Amo"?
Rashi: It is to take vengeance for His nation. This is like "Ki Yadin Amo v'Dam Avadav Yikom" (Devarim 32:43).
