Whether Obama started it or not is irrelevant in this context, which is how Trump is dealing with the construction of the pipeline. As for environmental impact, getting into a circular, endless argument with you is sooooo tempting, but utterly pointless of course.

"your attempts to show Trump in as poor a light as possible"

Just trying to shed a little light into the shallow but impenetrably dark abyss that is the orange one's mind to try to understand motivation in this case.

But we know the motivation. Sub-President Trump is an uber-capitalist and climate denier, although unlike many climate/science deniers I think his reasons for denial are different... he's just not a very thoughtful individual and I suspect he could be persuaded if you pry the bile volcano that is President Bannon from his earlobe for a few moments.