In Psalm 126, we have two parallel verses, "Then they said among the nations: "The Lord did great things for these."  The Lord did great things for us.  We were glad (v. 3-4)."   The news of the great things G-D did for us spread to the nations, who proclaimed "the Lord did great things for the children of Zion."   The Da’at Mikra understands "the Lord did great things for these" as an expression of contempt.  The nations are shocked by the fact that Hashem did great things for the Jewish nation, a nation seen as contemptible.  The children of Israel then respond: "The Lord did great things for us."   We repeat the same words as if to say, "You have indeed spoken the truth.  Hashem has and does great things for us."   The latter half of the verse, "we were glad", parallels the words-  "we were like those who dream" at the beginning of the psalm.   Initially we were like dreamers, but after hearing the words of the nations we realized that this was not a dream but a reality.  We have been validated by hearing confirmation by our enemies who have no reason to distort the truth in our favor.  If they can see Hashem’s goodness on our behalf, then we are surely glad.