The London-based human rights organisation said today that prosecutors at Pfc Bradley Manning's court-martial haven't proven he aided the enemy.
A conviction requires proof that Manning knew the material would be seen by America's enemies on the WikiLeaks website.
Aiding the enemy is the most serious of 21 contested counts. It carries a possible life sentence.
Amnesty says prosecutors also should drop other charges unsupported by evidence.
Manning's lawyers have asked the military judge to acquit Manning on seven counts, including aiding the enemy, due to a lack of incriminating evidence. She will hear both sides' arguments Monday at Fort Meade, near Baltimore.