* I don't think many people are so extreme as the 'opposite of nationalism'. In fact, I think that nationalism (or more broadly, the basic tribalistic instinct) has such an natural, powerful pull on people that it will never need to be encouraged.
* I think it's great that many people moderate their nationalistic instincts now with reason and compassion. Not only did millions get slaughtered for the nation (or the race, or whatever), but they murdered millions more. The moderation of 'tribalism' with liberal ideas might be one of the most important changes in humanity's history.
* Unfortunately, I see nationalism and other tribalistic ideas as rapidly spreading again. It's promoted by state school boards in Texas and Oklahoma; many national leaders push it, for example in Russia, China, Japan, and South Korea (off the top of my head). Outright religious 'tribalism' is rampant in Europe and, to a degree, the U.S. The EU's unity is under serious strain, Scotland almost succeeded from the UK, and parts of Spain and other countries are discussing the same. Nobody stands up for the value of the liberal ideas and institutions that kept the peace since WWII.
Arguably, there are two kinds of political movements (and leaders), those that bring people together, and those that play on tribalistic instincts and divide them. One kind has much better results for humanity.