Why should he tell this to Zerubavel?
Radak: He should know that the wars will not lower his level. Rather, he will be raised greatly - "Ekachacha... v'Samticha ka'Chosam" (23).
Malbim: The sprouting of Beis David depends on arrival of the time of the true Ge'ulah. The war of Gog and Magog must come first. During the Bayis Sheni, there was no king from Beis Yehudah. Zerubavel was called Pachas Yehudah, and not king. The Navi saw that the time of the Tikun had not come; the generation did not merit it via their deeds. He informed Zerubavel that even so, Hashem's word will come, at another time - if they merit, "Achishenah" (Yeshayah 60:22 - I will bring it early); if they do not merit, "b'Itah" (in its time) (Sanhedrin 98a).
How will Hashem be Mar'ish Shamayim and Eretz?
Radak #1: This is a Mashal for the wars that were in the times of Malchus Paras. They were in the days of this Daryavesh, or his son Artachshasta who ruled after him.
Radak #2: This Nevu'ah refers to the Bitul pf Malchus Paras, and the start of Malchus Yavan - "v'Hafachti Kisei Mamlachos" (22). This was 34 years after Binyan ha'Bayis. So Chazal said (Avodah Zarah 9a), the Parsim ruled for 34 years of Bayis Sheni. The Yevanim ruled for 180 years, the Chashmona'im for 103 years, and Hordus and his seed for 103 years. These comprise the 420 years that Bayis Sheni stood.
Malbim: Refer to 2:22:1:2.
