We read in Isa 2:2-4, "It shall come to pass in the latter days that the mountain of the house of the LORD shall be established as the highest of the mountains, and shall be lifted up above the hills; and all the nations shall flow to it, and many peoples shall come, and say: "Come, let us go up to the mountain of the LORD, to the house of the God of Jacob, that he may teach us his ways and that we may walk in his paths." For out of Zion shall go the law, and the word of the LORD from Jerusalem. He shall judge between the nations, and shall decide disputes for many peoples; and they shall beat their swords into plowshares, and their spears into pruning hooks; nation shall not lift up sword against nation, neither shall they learn war anymore." Generally the beating of swords into plowshares is taken to be the fruit of salvation in which we are changed into peacemakers from the haters that we are by nature. I assume that most see Isaiah as prophesying about the gospel age and with these two views I agree. Yet there are some difficulties.
I have heard this and similar passages, see Micah 4:3, as referring to a golden age on earth in which the gospel has so swept over the earth that entire nations, if not all nations, no longer will engage in war and there will be universal peace. Others see it as referring to a millennial age in which Christ will rule from Jerusalem and enforce peace on earth if you will.
It seems that by understanding the term "nations" as having to refer to actual national entities we run into some problems. First of all the gospel is never said to affect nations, only individuals. We don't preach the gospel to governments but individual sinners. The word for nations in Isaiah basically is a word used for Gentiles and is telling us that all people groups will benefit from the gospel. Some believe that Matthew 25:32 is referring to nations also, "Mat 25:32 Before him will be gathered all the nations, and he will separate people one from another as a shepherd separates the sheep from the goats." But this word also is "ethnos" and refers to all people groups along with the Jews. The ESV I think rightly goes on to say he will separate people even though the KJV properly translates it "them" because it does refer back to the nations. The reason is because they are then referred to as sheep and goats which is always a term of individuals.
Realistically can we ever expect a nation and in particular its government to be so "Christianized" that it will disband its army? Would it not immediately be overthrown by another nation? Arguably America was the best example of a Christianized nation and yet there was never any hint of anything so foolish as to disband our military. In fact, we quickly produced the finest military on earth. The only way this prophecy could apply to nations in this sense is if it was applied almost universally. It would be saying that Christ would bring peace on earth through the gospel and not by his return.
Yet another problem is that Jesus himself said along with the writers of the New Testament that things would get worse and worse on the earth, not better and better. "There will be wars and rumors of war". It is hard to read The Revelation and see a golden age on earth until after Christ makes a new Heaven and a new Earth. Paul in 2 Tim. 3 says that the last days will be one marked by all kinds of sinfulness not men who love and are at peace with each other but self-lovers.
So instead of seeing this as a prophecy of a golden age before Christ comes back; some wishful idea that the gospel will usher in a utopia, perhaps there is a simpler way to understand it. Why can't it just be a reference to all ethnic groups or all kinds of people from all over the earth? So it does refer to the effect of the gospel but on individuals because the gospel doesn't apply to and can't save and change governments. We might fight the wars of our governments but in our own lives we love, we don't hate. And such people exist among all nations or people groups.
This might help explain this passage in Isa. 11:9-12, "They shall not hurt or destroy in all my holy mountain; for the earth shall be full of the knowledge of the LORD as the waters cover the sea.
In that day the root of Jesse, who shall stand as a signal for the peoples--of him shall the nations inquire, and his resting place shall be glorious. In that day the Lord will extend his hand yet a second time to recover the remnant that remains of his people, from Assyria, from Egypt, from Pathros, from Cush, from Elam, from Shinar, from Hamath, and from the coastlands of the sea. He will raise a signal for the nations and will assemble the banished of Israel, and gather the dispersed of Judah from the four corners of the earth."
The knowledge of the Lord can cover the earth in that it is known among all groups of people. It doesn't have to mean that every individual is a Christian or that every nation is Christian and as a nation seeks God. Just before verses 9-12 in Isaiah 11 it refers directly to the change the gospel will make in individuals who are in the promised Kingdom of the Messiah, "The wolf shall dwell with the lamb, and the leopard shall lie down with the young goat, and the calf and the lion and the fattened calf together; and a little child shall lead them. The cow and the bear shall graze; their young shall lie down together; and the lion shall eat straw like the ox. The nursing child shall play over the hole of the cobra, and the weaned child shall put his hand on the adder's den."
Admittedly these are not among the easiest to understand passages and subjects in the Bible, but it seems to me that this position avoids difficulties that we don't need to have.