Lawful permanent resident or green card holders can enlist and serve in the U.S. Military, and can apply for naturalization or U.S. citizenship through their military service.
Judge held that the Department of Defense likely violated the federal Administrative Procedure Act after it implemented a policy discriminating against lawful permanent resident enlistees. Judge's ruling finds that the Defense Department provided no rational justification for the policy change, stating that it provided no evidence indicating that lawful permanent resident enlistees posed more of a risk than U.S. citizens.
Read more here.
Please note that in October 2017, the Department of Defense issued new policies that impact lawful permanent residents and other non-U.S. citizens in the military. The ILRC's practice advisory discusses how these policies affect those who seek to enlist, and those who currently serve in the military, including in the Reserve Components.
The DoD policy changes will not affect MAVNI enlistees because the MAVNI program was suspended in October 2016. No one has been able to enlist in MAVNI since that time.
Before making a decision to enlist and to apply for naturalization, please review the practice advisory (dated 03/2018 - will be revised soon) and consider that under new rules "expedited" naturalization may not be much faster than a naturalization under a default rule.