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I'd ask you to consider that the situations in Syria and Ukraine are not nearly as straightforward as the US would have us believe; indeed, the US and Israel are, as usual, the main instigators.
Regardless, Corbyn hasn't "crossed any lines" - he certainly hasn't publicly stated that it's OK to cut a civilian population's water supply as collective punishment, for example. Corbyn takes a more considered, nuanced, sensible view on world politics, which unfortunately doesn't play well with our right-wing press's simplistic "good guy, bad guy" gov-sponsored narrative. This is why Corbyn was smeared - he stands up for what's right, even if it means going against the US and Israel.