The atomic scale lithiation mechanism of individual SnO2 nanowires in a flooding geometry was revealed by in situ transmission electron microscopy. The lithiation was initiated by the formation of multiple stripes with a width of a few nanometers parallel to the (020) plane traversing the entire wires, serving as multiple reaction fronts for later stages of lithiation. Inside the stripes, we identified a high density of dislocations and enlarged interplanar spacing, which provided an effective path for lithium ion transport. The density of the stripes increased with further lithiation, and eventually they merged with one another, causing a large elongation, volume expansion, and the crystalline-to-amorphous phase transformation. This lithiation mechanism characterized by multiple stripes and multiple reaction fronts was unexpected and differed completely from the expected core-shell lithiation mechanism.