Shir Hashirim has the words השבעתי אתכם, that I made you swear, three times.
The Talmud at the end of מסכת כתובות says that this repetition refers to the “three oaths” that were made between Israel and the nations during Israel’s long and bitter exile.
The three oaths were that Israel would not rebel against those nations that were hosting them. They allegedly agreed that they would not return to Israel in large numbers, but would wait for Mashiach. And the third oath was that the nations of the world would not oppress us too much.
While some use these oaths as an excuse not to make Aliya, others claim that the deal is off. This is because they did oppress us too much.
Once they broke their part of the deal, Jews are certainly allowed to make Aliya from all over the world.