Jamieson, Fausset and Brown on Zechariah 1:3

3. saith the Lord of hosts--a phrase frequent in Haggai and Zechariah, implying God's boundless resources and universal power, so as to inspire the Jews with confidence to work.

Turn ye unto me ... and I will turn--that is, and then, as the sure consequence, "I will turn unto you" (Mal 3:7; Jas 4:8; compare also Jr 3:12; Eze 18:30; Mi 7:19). Though God hath brought you back from captivity, yet this state will not last long unless ye are really converted. God has heavier scourges ready, and has begun to give symptoms of displeasure [Calvin]. (Hag 1:6).