And it was GIVEN TO HIM to MAKE WAR with the SAINTS and to overcome them;
This is the second reason (see verse 5) for the existence of the "Sea Beast".
The SAINTS referred to in this verse are those people after the rapture event who realize who Christ is and the price Christ paid for their sins. These SAINTS make a commitment to follow Jesus Christ. Perhaps they heard the two witnesses (Revelation 11) or someone from the 144,000 sealed Jews talked to them (Revelation 7) or they dug out and read their Bible or they may have pulled up this web site. Some way or other, they make a commitment to Jesus Christ.
While the "Sea Beast" is GIVEN the great power to make war (not necessarily military but perhaps economic, health, or legally with euthanasia laws (Daniel 7:25)) to overcome the SAINTS (Job 1:6-12) and kill them, the SAINTS do "win" in the end as Revelation 7:9,14 points out.
and AUTHORITY over every tribe and people and tongue and nation was GIVEN TO HIM.
This is the third reason (see verse 5) for the existence of the "Sea Beast".
The devil tried to get AUTHORITY over all the nations before in Luke 4:5-8. He didn't succeed then, but this time he accomplishes it because he is permitted to have this authority (for a short time).
This will be the last attempt to establish a devil-controlled world empire. The first attempt by Nimrod (Genesis 10:6-14; Genesis 11:1-9) resulted in the "tongues" referred to in this verse. But, as this verse points out, even the diversity of spoken languages will be overcome by the "Sea Beast".