A warning at The National Interest:
COULD A U.S. response to Russia’s actions in Ukraine provoke a confrontation that leads to a U.S.-Russian war? Such a possibility seems almost inconceivable. But when judging something to be “inconceivable,” we should always remind ourselves that this is a statement not about what is possible in the world, but about what we can imagine. As Iraq, Libya and Syria demonstrate, political leaders often have difficulties envisioning events they find uncomfortable, disturbing or inconvenient.
As a general principle, the future can be expected to deliver the ‘unthinkable’. (It’s in our nature to forget, very quickly, how much of that happens.)