Why does it say "Lachen"?
Radak: Because Amon seized Gad's land, the plunderer will come until the capitol, Rabas Bnei Amon, and destroy it and its suburbs.
Since they acted as if Yisrael do not have heirs, Midah k'Neged Midah, Amon will not leave heirs, and Yisrael will inherit them. (It seems that this is Malbim's intent - PF.)
When will Yisrael dispossess the one who took its land?
Radak #1: This will not be at the time of this punishment [of Amon] via Nebuchadnetzar, for then Yisrael were in exile! Rather, because Amon took Eretz Gad, there will be a time when Yisrael will be Yoresh Amon, who was Yoresh Yisrael. This will be at the end of days - "u'Vnei Amon Mishmatam" (Yeshayah 16:2).
Radak #2: Perhaps this was via Pras and Madai; also they struck Amon, like it says "Hineni Nosnach li'Vnei Kedem l'Morashah" (Yechezkel 25:4). Yisrael ascended from exile via Koresh Melech Pras, and he gave to them authority in Eretz Bnei Amon. Initially Amon fell to Melech Bavel, like the other nations, and afterwards they rebelled, like Tzidkiyah rebelled.
