2. cup of trembling--a cup causing those who drink it to reel (from a Hebrew root "to reel"). Jerusalem, who drank the "cup of trembling" herself, shall be so to her foes (Isa 51:17, 22; Jer 13:13). CALVIN with the Septuagint translates, "threshold of destruction," on which they shall stumble and be crushed when they attempt to cross it. English Version is better.
      both against Judah--The Hebrew order of words is literally, "And also against Judah shall he (the foe) be in the siege against Jerusalem"; implying virtually that Judah, as it shares the invasion along with Jerusalem, so it shall, like the metropolis, prove a cup of trembling to the invaders. MAURER with JEROME translates, "Also upon Judah shall be (the cup of trembling); that is, some Jews forced by the foe shall join in the assault on Jerusalem, and shall share the overthrow with the besiegers. But Zec 12:6, 7 show that Judah escapes and proves the scourge of the foe.

JFB.