The United States, Iran and five other world powers on Thursday reached a preliminary deal designed to contain Iran’s nuclear program, one that would restrict Iran’s ability to enrich uranium and subject it to international inspections, but which also would gradually remove harsh economic sanctions from the Islamic Republic.

The agreement, if it holds, could prove to be President Barack Obama’s biggest foreign policy achievement, a success stemming from his conviction that it’s worth engaging even longtime adversaries such as Tehran. But while the framework won praise for its surprising level of detail, it failed to quell sharp skepticism from many Republicans as well as Israeli Prime Minister Benjamin Netanyahu.

“It is a good deal,” Obama declared Thursday, calling it “a historic understanding with Iran which, if implemented, will prevent it from obtaining a nuclear weapon.