This is the word of the Lord to Zerub′babel: Not by might, nor by power, but by my Spirit, says the Lord of hosts. (Zech. 4:6)
The word חַיִל is usually translated as "might" here. But more often it means "army," by a count of 56x to 6x in the KJV. This leads to the question of whether Zerubbabel is being warned against the use of force.
Arguing against this is the context of the verse, which has to do with Zerubbabel's role in rebuilding the Temple. The next verse states: "What are you, O great mountain? Before Zerub′babel you shall become a plain; and he shall bring forward the top stone amid shouts of ‘Grace, grace to it!’”
The military interpretation of the verse, however, is supported by two other factors: Zerubbabel's later history and the prophecy about him in Haggai 2. In Ezra 4, we learn that Zerubbabel's position was undermined by opponents whom he had excluded from the rebuilding process:
“To Ar-ta-xerx′es the king: Your servants, the men of the province Beyond the River, send greeting. And now be it known to the king that the Jews who came up from you to us have gone to Jerusalem. They are rebuilding that rebellious and wicked city; they are finishing the walls and repairing the foundations... We make known to the king that, if this city is rebuilt and its walls finished, you will then have no possession in the province Beyond the River.” (Ezra 4:11-16)
Thus, Zerubbabel's activities were characterized by his detractors as rebellious, with the implication that he would soon restore Judah's independence. This image may have been strengthened by the prophecy of Haggai:
Speak to Zerub′babel, governor of Judah, saying, I am about to shake the heavens and the earth, and to overthrow the throne of kingdoms; I am about to destroy the strength of the kingdoms of the nations, and overthrow the chariots and their riders; and the horses and their riders shall go down, every one by the sword of his fellow. On that day, says the Lord of hosts, I will take you, O Zerub′babel my servant, the son of She-al′ti-el, says the Lord, and make you like a signet ring; for I have chosen you, says the Lord of hosts. (Haggai 2:221-23)
To answer the main question, several factors may be considered: Considering Haggai's prophecy, were Zerubbabel's detractors correct in their view that he was planning to make Judah independent of Persia? Was the prophecy of Zechariah ("not by might/army") a warning to Zerubbabel to avoid any hint of reconstituting Judah's military strength, or was it a poetic way to emphasize spiritual matters (the Temple) and thus encourage Zerubbabel to finish the building work? Finally, what happened to Zerubbabel; and is his disappearance from the biblical record related to his detractors raising suspicions about him in the Persian court?