Chapter 15 in many ways is a lead-up to chapter 16: the angels make their appearance, but it is in chapter 16 the the bowls of God's wrath are to be poured out. So what do we make of these plagues? Are they literal or symbolic? If you've been reading, you'll know that I think they're literal.
"The greatest plagues of judgment of which we read in the past were those poured out upon ancient Egypt. They were literal plagues, which happened according to the terms in which they are recorded. These seven plagues are the consummation of God’s judgment plagues, including in them all that have gone before, and rendering in final and intensest perfection what was previously rehearsed on a smaller scale, preliminary to the great performance. What the preparatory rehearsal was, that must the final rendering be. The last plagues must therefore be literal too." 
The bowls will be poured out in the second half of the tribulation, known has the Great Tribulation, and will be one of the last things to happen before the tribulation is ended. Things climax in a final battle, which some say would be a kind of World War 3 (if it happens in our day). Demons will go forth and stir the nations against God.
"Since the nations of the whole world will be effectively under a single government during the Tribulation, how it will be possible for there to be a world war at the end of the Tribulation?
One answer suggests that the armies, all loyal to the beast, are gathered in anticipation of the Second Coming of Christ to make war against Him and His heavenly armies (17:14; 19:19)." 
Another option is that this war is made against all believers still alive. There will be people that turn to Jesus during the Tribulation - that we already know. Many of them will die, particularly when making the decision not to take the mark of the beast. But it is very possible/probably that there will be many still alive that have been in hiding, most notably the 144,000 hiding in the desert that we read about before. There is definitely conjecture here, but the battle is certain. This could be a single battle or a collection of them, but my feeling is that it is a single battle which is ultimately put to a stop by the pouring out of the 7th bowl that destroys everything, causing the fall of Babylon (referring either to an actual city that is the seat of the world government of the antichrist, or symbolically of the forces that band against God). The next chapters will dwell upon these events and the fall of Babylon.
 Ibid, Re 16:12–16.