Matthew Poole and continuators on Joel 2:21

21 ¶ Fear not, O land , be glad and rejoice : for the LORD will do great things .

Verse 10. The Land is said to quake at the approach of this devouring Army, now at the approach of mercy 'tis encouraged, and commanded not to fear, by the same figure and in the same sense, it should not fear now as before it did quake.

This tells us that fear not, was a Meiosis, shout for joy express the greatest confidence.

Who is your God whom you seek, who by me assureth you 'tis his gracious purpose to do great things for your good.

Either this explains that last clause, ver. 20. or is Antitheron to it, the locusts, God's messengers and Servants did great things against, but now God will recall these and do great things for Israel.