Transformers Siege is the first part of the War for Cybertron trilogy of toylines. It focused on Cybertronian alternate forms and cartoon inspired robot forms for the characters, and reintroduced Micromasters into the line.
Takara-Tomy released Transformers Siege in Japan without any alterations to the toys. For more information on the Japanese release please see Transformers Siege (Japan).
A number of retools of Siege molds were released as a part of Transformers Generations Selects. Many of these complemented toys or completed groups of characters released in Transformers Siege.