The nations in the U.N. who voted in favor of a Jewish state did not include the nation ruling over the land, the British. The British abstained in the vote and did nothing to help carry out the U.N. resolution. In the end, the Zionists did have to fight for their land, first against the local Arabs (Palestinians) and then against the surrounding nations. That is definitely "with a strong hand" and a rebellion against the nations. The fact that they had a recommendation to do so from other nations around the world who were not involved in the conflict means nothing halachically speaking. And the 1948 war was not a fight to defend a piece of land; it was a fight to get a piece of land.

Imagine that all the countries in the UN except America voted to give the Jews the state of New York for a country of their own. And the Jews held a meeting and declared independence in the state of New York. And then the U.S. army came to fight them. Would it then be self-defense to fight back against the U.S. army?

Furthermore, not everyone agrees that going up "as a wall" means by military means. The Avnei Nezer is the only one who says that. Others (Yefeh Kol, Ahavas Yonasan) understand it as any mass immigration.

Also, do not forget about the other oath, which prohibits forcing the end. Founding a state before the coming of moshiach certainly falls under that category. Even the Avnei Nezer only says that the oath against "going up as a wall" becomes permitted when the nations give it to us, which would mean that mass immigration is permitted according to him, but not founding a state.

Furthermore, the borders of the Jewish state proposed by the UN are a far cry from the borders actually conquered by the Zionists. The U.N.'s Jewish state was made up of a thin strip along the coast, the Negev desert, and a strip in the northeast. These three pieces are barely connected. And all of Jerusalem and its environs were to be deep within the Arab state.

If I walk into your house and declare it mine, and, when you resist me, I fight back, is that self-defense? Who is the aggressor? I am, of course. Declaring someone else's land mine is an act of aggression. True, here there was no sovereign power from which the Israelis took the land. The previous government was the British, and they left voluntarily. But my point is that self-defense is not defined by who attacks physically first.

There are actually three possibilities. 1) If someone attacks my house, which has always been mine, then he is the aggressor and I am just defending myself. 2) If I walk into someone's house and declare it mine, then I am the aggressor. 3) If there is an ownerless house and two people want it, the fact that one of them happens to be the first one to declare it his does not make his opponent the aggressor. They are both equally aggressors. This last case is the analogy that best describes 1948.

The UN resolution didn't make the Zionists into the defenders, because as I said, the UN was only making a recommendation. It wasn't their country.

Your Zionist friend would argue that the Arab countries all agreed to the rules of the UN, and therefore although they voted against a Jewish state, it is as if they agreed to it. But even if that were true, it would be irrelevant because the bottom line is that there was a war and Jews would not have gotten control of the country without a war. Taking the land through war is prohibited by halacha.

To put it another way: If you think agreeing to abide by the rules of the UN amounts to agreement to give the Jews the land, so be it, but even if a ruling power really did agree to give the Jews its land and then later retracted its offer, and the Jews had to fight for it, that is “with a strong hand”.