Explain the core conflict of the 'Secret Invasion' storyline.

'Secret Invasion' revolves around the Skrulls, an alien race capable of shapeshifting, secretly replacing key figures in the Marvel Universe with their own agents. The core conflict is the paranoia and distrust that ensues as heroes and villains alike struggle to identify who is genuine and who is a Skrull imposter. The story explores themes of identity, betrayal, and the fragility of trust within a community. It led to significant character deaths and reshaped the political landscape of Earth.