Why Euphrates dries up to prepare the way for the kings from the east.

The Kings from the East are Christ and His angels at His return. The Beast is false Christianity. Drying up of the Euphrates means that the people’s support for false Christianity dries up. But it then receives renewed strength from demons, performing signs. Through this supernatural support, it gathers the kings of the whole world for Armageddon.
The people with the Mark of the beast are the modern Pharisees. They are Christians for whom traditions are more important than truth. In reality, Satan is their god. Through the first five plagues, they come to realize that they are serving Satan, but they do not repent, for they have become hardened. When Satan sends his demons, these modern Pharisees willingly accept this new power with full knowledge that this power is from Satan. This reveals their true nature and it confirms God’s judgment.