Approval of the XL Pipeline will lead directly to increased CO2 emissions associated with climate change, including drought and crop failure.
Nations with inadequate food risk becoming politically stressed, and of their governments collapsing - when people are desperate or undernourished, they blame their governments, which they see as responsible for or unable to solve their problems.
Modern "state failures" - i.e., revolutions, violent regime change, collapse of authority and genocide - are therefore directly related to environmental pressures. Such situations force the United States to respond with military or other direct aid which can cost hundreds of millions of dollars.
The White House should take the long term view and halt the XL Pipeline on national security grounds.