Jamieson, Fausset and Brown on Joel 3:10

10. Beat your ploughshares into swords--As the foes are desired to "beat their ploughshares into swords, and their pruning hooks into spears," that so they may perish in their unhallowed attack on Judah and Jerusalem, so these latter, and the nations converted to God by them, after the overthrow of the antichristian confederacy, shall, on the contrary, "beat their swords into ploughshares, and their spears into pruning hooks," when under Messiah's coming reign there shall be war no more (Is 2:4; Ho 2:18; Mi 4:3).

let the weak say, I am strong--So universal shall be the rage of Israel's foes for invading her, that even the weak among them will fancy themselves strong enough to join the invading forces. Age and infirmity were ordinarily made valid excuses for exemption from service, but so mad shall be the fury of the world against God's people, that even the feeble will not desire to be exempted (compare Psa 2:1-3).