After the sin of the spies, Moshe Rabbeinu warned those who regretted their original lack of faith that they should not attempt to go into Eretz Yisrael by force: vehi lo sitzlach, it will not be successful. Rabbi Tzadok Hakohein of Lublin (Tzidkas Hatzadik 46) comments, “Now it will not succeed, but there will be another time when it will succeed. That will be in the time of the footsteps of moshiach.”
Reb Tzadok doesn't say it will be permitted, only that it will be successful. There is a big difference. He writes that the Jews who attempted to invade the land knew that their act was against the will of Hashem, but justified it based on the statement of Chazal, "All that the host tells you to do, you must do, except for leaving” (Pesachim 86b). They understood this to mean that for the sake of coming close to Hashem, one may sometimes violate the command of Hashem. We need not listen when He tells us to leave Him. Despite these good intentions, they were punished severely for their sin. But Moshe said to them, "And it will not succeed" - this time it will not succeed - hinting that there would come a time when such a sin would have success. “In the Footsteps of the Moshiach, chutzpah will increase” (Sotah 49b). That is the time when such a brazen idea – to conquer the land in violation of Hashem’s command – will meet with some success.
Furthermore, if you read Reb Tzadok carefully you will see that he is not even talking about the physical conquest of Eretz Yisroel. He interprets the entire story of the spies and the invaders in accordance with the Zohar (3:161) which takes it as a metaphor for Torah. The Jewish people's punishment after the sin of the spies was that they should not get Torah, in accordance with the rule (Tehillim 50:16), "To the wicked, G-d says, Why do you speak of my laws?" In the time before the coming of moshiach, the wicked will brazenly try to learn Torah against Hashem's will. Eventually, the Torah will make them better people and Hashem will be happy with them.