Detrimental to Israeli security
Unilateral annexation violates Israel’s commitments under the 1993 Oslo Accords and severely undermines an already weak Palestinian Authority (PA). It will threaten Israeli security cooperation with the Palestinian Authority, either through an official cessation of cooperation or through a de facto reality when Palestinian security forces no longer show up to work. If Palestinian security coordination with Israel ceases to be effective, Hamas and other violent extremists will be well positioned to fill this vacuum, and the IDF will need to scale up its already extensive policing and security presence in the West Bank, leaving Israel potentially vulnerable on other fronts in need of major attention and reinforcement.
This scenario could also lead to a full-scale reoccupation of the entirety of the West Bank, including all Palestinian population centres currently under PA administration -- like Ramallah, Nablus, and Jenin. The outlined domino effect could trigger a violent response from Palestinians who have given up hope for a diplomatic process, paving the way for an era of greatly incresed extremism and destabilisation in the West Bank and Gaza — leaving Israel with no exit strategy.
Furthermore, unilateral annexation threatens decades-long peace treaties and stability with Egypt and Jordan, discussed further under the section on regional strategic relationships.