Gemara

24b (Mishnah): In Yehudah, an Am ha'Aretz is believed about (Taharah of his) wine and oil the entire year...

25a - Inference: They are not believed in Galil.

Question: What is the reason?

Answer (Reish Lakish): There was a strip of land of Nochrim between Galil and the Mikdash (Taharos brought to the Mikdash would become Tamei).

Question: One could bring them in a box and avoid the Tum'ah!

Answer: Our Mishnah is like Rebbi:

(Beraisa - Rebbi): One who enters Chutz la'Aretz in a box becomes Tamei;

R. Yosi b'Rebbi Yehudah says, he remains Tahor.

Gitin 8a (Beraisa): Surya is like Eretz Yisrael in three ways, and like Chutz la'Aretz in three ways:

Its dirt is Tamei like that of Chutz la'Aretz... one may enter Surya and remain Tahor.

Question: If its soil is Tamei, how may one enter and remain Tahor?

Answer: He enters in a box;

(Beraisa - Rebbi): One who enters Chutz la'Aretz in a box becomes Tamei;

R. Yosi b'Rebbi Yehudah says, he remains Tahor.

Even Rebbi is Metamei only in Chutz la'Aretz proper. There, Chachamim decreed Tum'ah on the soil and the air. In Surya, they decreed only on the soil.

Rishonim

Rambam (Hilchos Tum'as Mes 11:1): At first, Chachamim decreed about the soil of Chutz la'Aretz, that it is like a Beis ha'Peras. One became Tamei only if he walked on it or touched or moved it. Later, they decreed that the air is Metamei, even if one did not touch or move it, just he stuck his head and majority in the airspace. Also, if the interior of a Keli Cheres or the majority of another Keli was entered into the airspace, it becomes Tamei.

Rambam (2): The Tum'ah of the air is less than that of the soil... One who becomes Tamei from the soil needs Haza'ah on days three and seven, but one who became Tamei through the air needs only Tevilah and ha'Arev Shemesh.

Answer #3 (Likutei Halachos, Nazir 21a Zevach Todah): The Rambam holds that Tum'as Avir does not obligate Haza'ah. Ohel Zaruk is not considered an Ohel, so one who goes in a box is like one who walks on the ground, so he needs Haza'ah.

Rambam (5): One who walks on mountains or rocks of Chutz la'Aretz is Tamei for seven days. One who goes in the sea or a place where the sea goes when it is stormy is not Tamei due to the soil, but he is Tamei due to the air. One who enters Chutz la'Aretz in a box flying in the air becomes Tamei, because an Ohel Zaruk is not considered an Ohel.

Rambam (6): The soil of Surya is Tamei, like Chutz la'Aretz. Its air is Tahor; Chachamim did not decree on the air. Therefore, if Surya bordered Eretz Yisrael without Chutz la'Aretz, a cemetery or Beis ha'Peras in between, one may enter Surya in Taharah, in a box, as long as he does not touch the soil. Similarly, if Chutz la'Aretz borders Eretz Yisrael without Tum'ah in between, one may check it, and it is Tahor.

Rebuttal (Ra'avad): The Tosefta says so only regarding areas that are enveloped (within Eretz Yisrael).

Defense (Kesef Mishneh): The Tosefta (Ohalos 18:1) says that one may enter b'Taharah if it is within a furrow of Eretz Yisrael. The Rambam understands that we did not decree on it, but it must be checked.

Mishneh l'Melech (DH u'Mah she'Chosav): The Rambam holds that one can enter Surya close to Eretz Yisrael without checking, or Chutz la'Aretz close to Eretz Yisrael by checking. After checking he may walk directly on the ground.

Questions (Tosfos Chagigah 25a DH she'Retzu'ah): If a strip of Chutz la'Aretz interrupts, how could Efer Chatas be brought to Galil? How could people of Galil come for the Regel without becoming Tamei for seven days?

Answer #1 (Tosfos): There was a thin (Tahor) road from Yehudah to Galil. It sufficed to transport Efer Chatas, but it was too narrow to bring wine and oil.

Answer #2 (Mishneh l'Melech 1, DH Od Kosvu): The Rambam holds that one who goes through in a box does not become Tamei for seven days.

Answer #3 (Rav Ransborg 1): People can ride on doors on bulls, according to the Rashba (above). The Maharsha (Eruvin Reish 31a) says that Chachamim did not rely on this to transport Kodshim b'Taharah, for it is not common. They were not stringent about the Mitzvah of Aliyah l'Regel.

Question (Sha'ar Efrayim 36, cited in Gilyon ha'Shas): What is Tosfos's question? Perhaps people came a week before the Regel and did Haza'ah on days three and seven! Tosfos did not ask about Bikurim; perhaps they brought fruits that were not Huchshar. Ha'Rav Heshel says that they brought Bikurim in a Keli with a Tzamid Pasil. It is only regarding Kodshim that this is not sufficient.

Rebuttal (of Rav Heshel - Pri Etz Chayim, Bikurim Siman 38): In some ways Bikurim are unlike Kodshim, e.g. a Mechusar Kipurim may eat it. In some ways they are like Kodshim, e.g. Me'ilah applies to them. It seems that after Kri'as Shem they are like Kodshim. Tana'im (Makos 18b) argue about after the Kohen gets them, whether they are like Kodshim received from Shulchan Gavoha, or if they are Matanos Kehunah like Terumah (that had Dinei Kodshim as long as they were needed for Avodah).

Mishneh l'Melech (DH u'Mah she'Chosav): If there was Tum'as Ohel between Eretz Yisrael and the strip, they could not transport Taharos in any case! Rather, we must say that there was no Tum'as Ohel in between, and there was a decree on Avir. Nevertheless, they could pass b'Taharah by checking! It seems that it was not really Eretz ha'Amim; it is bizarre that Eretz ha'Amim should separate between Yehudah and Galil! Rather, the strip was Tamei due to Mador ha'Amim (Nochrim lived there).

Poskim

Shulchan Aruch (YD 369:1): A Kohen is commanded not to become Tamei through a Mes or all Tum'os that come from it...and he is forbidden to become Tamei through Chutz la'Aretz.

Prishah (3, citing Maharshal), Bach (DH u'Mah she'Chosav v'Chol), Shach (2), Taz (4), and Chachmas Adam (159:1): This refers to when Eretz Yisrael was Tahor. Then it was forbidden to go to Chutz la'Aretz, where Nochrim bury their dead everywhere.

Rebuttal (Shevus Yakov 1:85): It is unreasonable to say that the Rif, Rosh, and Shulchan Aruch bring the law if it will not apply until Moshi'ach! It seems that Kohanim live in Chutz la'Aretz for the sake of income, like we allow going to Chutz la'Aretz to recover one's money. Not everyone can manage in Eretz Yisrael, and there are dangers. However, a Kohen in Eretz Yisrael should leave only for needs of income, learning and to get married. Tosfos says that the Gezeirah Shavah on Chutz la'Aretz was to stop Kohanim from leaving Eretz Yisrael! Kohanim in Chutz la'Aretz are already Tamei Mes mid'Rabanan, so they need not be careful of mid'Rabanan stringencies of Tum'as Ohel. This is why a Tosefta (cited in Tosfos Yevamos 61a DH mi'Maga) that permits Kohanim in Chutz la'Aretz to enter houses of Nochrim.

R. Akiva Eiger (DH v'Chol): Nowadays Kohanim are not careful about this. Perhaps they are lenient for the needs of livelihood. Alternatively, we are lenient because we are all Tamei nowadays.

Aruch ha'Shulchan (11): The Isur does not apply nowadays. The Shevus Yakov says that it does.