No one is against Israel. They are against the State of Israel.
It’s not so much a “concept” as it is history.
You see, the religious Jews have always been coming to Israel little-by-little for the past 2,000 years. They were not allowed to come in a massive “wall” of immigration for fear that it would upset those that lived in the land (I can give you sources for this ruling, but I have to look them up. Can anyone post them if you know where to find it off the top of your head? Thanks)
However, (and I’m leaving out much history that got up to this point) Theodor Herzl, an non-religious & misguided Jew (he wasn’t necessarily a bad person, he was just a little misguided in his earlier years) decided to bring about a massive wall of secular Jewish immigrants to Israel.
The problem with that massive wall was that it fueled much hatred from the “Palestinians” (truth be told, they hated us before it). And a lot of the issues that Israel is experiencing now with the Palestinians is because of this.
The history is pretty complex, but it actually goes back a lot further than 1948. Most of the history also began in Europe.
Either way, the early rabbis were opposed to the massive immigration, since they knew it would cause these same issues. However, after the rabbis saw they had no control, they decided to push for the immigration.
This is just skimming the surface. I studied if for about 15 hours a week for 5 months & had access to a library full of books written by both sides (the secular side & the religious side) & I can tell you that this subject is very enormous.