As Prime Minister Justin Trudeau and other NATO leaders head east to Lithuania this week for their annual summit, they'll be accompanied by some pretty pumped-up language and high expectations. Many veterans of the foreign policy circuit describe what's about to unfold on Tuesday and Wednesday as a "pivotal" moment for the alliance — maybe the most important since the end of the Cold War, or even in the alliance's nearly eight-decade history. There's one word no one has used — yet — to describe the summit: messy. The meeting in Vilnius is expected to solidify NATO's return to its…