Ex-PM Zoran Milanovic shakes up Zagreb's politics as its first EU presidency begins
Zoran Milanovic
Victory for challenger Zoran Milanovic could cause friction with right-wing government
Election run-off could shake Croatian politics as six-month presidency begins
Conservative president in close scrap with centre-left and nationalist challengers
Weak economy, Serbia tensions and migration crisis await country's new leader
Main parties seek allies as migration crisis and economic woes roil EU member
Centre-left ruling party and main rival will vie for smaller parties to form next government
Country has already erected fence almost the length of its southern border