Based on the Gemara in Kesubos (110b-111a).
There are three verses in Shir HaShirim (2:7, 3:5, 8:4) that refer to an oath placed upon the Jewish people not to "wake or rouse the love until it pleases." According to Rav Yehuda, in these verses are several oaths placed on the Jewish people:
- Not to immigrate to Israel from Bavel;
- Not to go up to the Land of Israel en masse;
- Not to rebel against the nations;
- Plus one on the nations that they shouldn't subjugate the Jews excessively.
According to Rebbi Zeira, only the latter three of the above are in these verses, but there are three more oaths as well:
- Not to reveal the end of days;
- Not to distance the end (by sinning);
- Not to reveal the secret (of Sod HaIbbur, or of the reasons of the mitzvos) to the nations
The important part for our discussion is that everyone agrees it's forbidden for the Jewish people to go to the Land of Israel en masse and not to rebel against the nations, and that according to Rav Yehuda, even immigration from Bavel is against the oath.
Now, the Rambam does pasken that you're not allowed to leave Bavel, just like you can't leave Eretz Yisrael (Hil. Melachim 5:12). Otherwise, though, this Gemara is left untouched by the major poskim (Tur, Shulchan Aruch, Rambam), assuming Ein Mishpat can be trusted.
In respect to seizing control of Israel (then-Palestine) from Britain in the late 40's, do any modern halachic authorities cite the above Gemara to state that there were any issues with that, or else cite the Gemara and then explain why it's not an issue?