Throughout most of our history, the international stage was viewed as a place of anarchy; sovereign states compete for security and power, looking out for their own self-interests. There is no higher authority to punish anyone for misbehaving, and every state wants to be at the top of the food chain, and will do whatever it can to get there. Alliances may be made, but no state can ever completely trust another state, because it is in no state's best self-interest to let anyone else know what all of its cards are. See Game Theory (no, not that one); specifically, the Prisoner's Dilemma. The simplest version of that is that two accomplices to a crime have been caught, and both have been offered a plea bargain. If neither one rats the other out, they will get a year's sentence. If one rats the other out, he or she will go free, but the other will get a ten year sentence. If they both rat each other out, they both get five. Essentially, in this simple version, Realists argue that it's in the best interest of one to rat the other out in most scenarios; if the other didn't rat them out, they go free. If the other did rat them out, their sentence is reduced by five years. Not ratting the other out risks having the other rat you out, so your sentence is the maximum.

In the last few hundred years or so, classical Liberalism became a thing. Liberal states would band together for the greater security of all (think League of Nations or the UN), and also to punish those who misbehave. Their worldview is that all states can work together, war can end, allies can be honest with each other, etc. It's been found that liberal states rarely go to war with one another (though obviously, they will still go to war with non-liberal states). So essentially, Liberalism believes the world is progressing past the need for Realism. (This is probably not the best explanation for either; I'm going by what I remember when I was reading this, heh...)

So yeah; which worldview sounds more realistic or reasonable to you? I think most of us would like to believe in the Liberalist Theory, but time and time again, and even now, the sovereign states of the world prove us wrong. In the US, our own government was spying on us, and was alsoi spying on our allies, like Germany. Clearly, we're not showing all our cards to the world; we're still looking out for #1. I think most of our citizens prefer it that way, anyway. Except for being spied on, probably. Also, even if we never openly go to war, we're still warring on each other economically, with all our sanctions. The Ukraine and Syria have been a bit like the proxy wars from the Cold War with present-day Russia, and China is getting really uppity in the South China Sea, making artificial islands so that they can claim more territory (also laying claim to the Spratly islands, islands in dispute by like 5 other countries who have a much more reasonable claim).

So yeah; I think the Realist theory is probably the more accurate way to describe the international scene, because we all suck.