“Who says that if we annex we need to give them all citizenship?”
If you annex and deny citizenship, you will truly have a South African Apartheid situation. You can’t have 1/3 of your population with no civil rights and expect to be called a democracy. Israel will be slapped with UN sanctions and Jews in the US will (correctly) support such. Another Intifada will ensue and the entire would will support the Arabs.
The US gave citizenship to French citizens when it annexed Louisiana, to Spanish subjects when it annexed Florida (and later Puerto Rico), to Texas citizens when it annexed Texas, to Mexican Citizens when it annexed the Southwest (including Mexicans who had just served in the Mexican Army against the US), to Russian subjects when it annexed Alaska, and to Hawaii citizens when it annexed Hawaii.