A federal judge has rejected the US military's request to dismiss a legal challenge to its training plans for the Northern Marianas.
The ruling is not a complete victory for opponents.
The military plans to relocate about 5,000 marines to Guam from Japan, and with that, it wants to use the island of Pagan and most of Tinian as bombing ranges.
But four environmental and cultural groups are challenging the decision, saying the Pentagon didn't properly consider any alternatives.
The military wanted the lawsuit dismissed, saying the court had no jurisdiction.
In a ruling on Friday, appeals court Judge Ramona Manglona agreed the court couldn't interfere in a political question regarding the relocation.
However, she said the court can rule on how the build-up itself is carried out, and that means the case alleging the military violated environmental planning laws can go ahead.
Ms Manglona has given both sides two weeks to file briefs in response to her ruling.