[Greece's Prime Minister Alexis Tsipras, right, welcomes European Council President Donald Tusk at Maximos Mansion in Athens Tuesday, Feb. 16, 2016. After decades of often unbridled expansion and increasing prosperity, the once-robust European Union is this week looking at its biggest challenge - crumbling from within, Tusk said. (AP Photo/Thanassis Stavrakis)]BRUSSELS (AP) — European Union chief Donald Tusk is making clear that the bloc's 28 leaders are heading into a summit this week that could fundamentally alter the political course of the continent.