This thesis is about Operation Forager and the capture of Marianas. The research problem is to identify the explaining factors that led to the US victory over Japan despite the latter’s prepared defence on the islands against the upcoming amphibious assaults. The study is a theory consuming case study where the three amphibious assaults to the Marianas function as analyse units with most different research design. Data was collected by document analysis and analysed by using the-matic analysis method due to the qualitative nature of collected data. Theoretical framework was established by applying theories of amphibious operations and success in land warfare. The analysis showed that the common factor for success on all three islands was tactical agility, rather than superior force ratio as opposed to earlier view. All identified factors are also linked to each other and isolating one specific factor for success is challenging.