The rambam (perek aleph hilchos terumas halachah hay) writes that kedushas oleh mitzraim is batel and there is no obligation of terumas and maseros in those areas (and it seems there is no obligation even derabonon). However the ramabam somewhere else (I forgot where but I think it was perek hay in hilchos malachim) says that Syria and the places that Dovid HaMelech conquered are obligated in terumas and maseros derabonon. Whats the difference? Why are the areas conquered by Dovid HaMelch more machmir then the places of Oleh Mitzraim (that were not later conquered by oleh bavel)? Thanks
A Kirsch, israel
1) The Rambam about Syria is also in the first chapter of Hilchos Terumos, in Halachah 3. If we look carefully at Halachos 3 and 5, we see that it seems that the Kedushah of Olei Mitzrayim is stronger than Syria. In the time of the first Beis ha'Mikdash, the Kedushah of Olei Mitzrayim was certainly d'Oraisa, while the Kedushah of Syria, even in the time of David ha'Melech, was not like Eretz Yisrael proper.
2) In Halachah 5, we see that when they returned to Eretz Yisrael in the time of Ezra, the Chachamim did not exempt -- from Terumos and Ma'aseros -- the places that the immigrants from Mitzrayim captured but the immigrants from Bavel did not capture. It seems from this, at first sight, that the areas conquered by Olei Mitzrayim were more Machmir than the areas conquered by David ha'Melech.
3) This is a big issue, which I will not go into at length at the present moment, but I will cite briefly other statements by the Rambam which do suggest that Syria is more Machmir than places of Olei Mitzrayim, as you wrote.
a) These sources are from the Rambam in Hilchos Ma'aser. The first is from 13:3, where the Rambam writes that when Chazal made the Gezeirag on Demai -- that someone who buys produce from an Am ha'Aretz must take Ma'aser from it -- they made this Gezeirah only on produce from the land which the immigrants from Bavel took possession of. We learn from this that there is no Din of Demai in places which only the Olei Mitzrayim conquered.
b) The second source is Hilchos Ma'aser 12:11, where the Kesef Mishneh writes that according to the Rambam, if someone buys produce in Syria and knows that it grew in a Jewish field, he must separate Demai. We learn from this that there is a prohibition of Demai in Syria, but there is no Isur of Demai in the places that only Olei Mitzrayim conquered.
4) The proof that I wrote above, that Olei Mitzrayim places are more Machmir than Syria in d'Oraisa questions, seems to be correct, but the d'Rabanan Halachos require a lot more research.
Kol Tuv,
Dovid Bloom