The great tribulation will be terrible and Jesus said if He did not shorten the days there would be no flesh alive which is three and one half years.
God will give the world 7 years to have their way,and cause all people that do not love Him to follow the beast kingdom,so He can end this sin business on earth.
But the tribulation is only for the last three and one half years of the 7 years period.
The first three and one half years repentance,and salvation is still available to the world,and there will be a unified religious system,and the ten horn kingdom,the world split in to ten sections with a leader in each section,and the first three and one half years are not part of the great tribulation,and that is when the 6th trumpet sounds,the religion of Islam and the world clash,and nuclear weapons devastate the Arab nations,and the man of sin plucks up 3 horns by the roots,and a third of the population of the world dies.
That war is so terrible they turn to the man of sin for the solution to have peace on earth,and the world follow the beast kingdom,and then the great tribulation against the saints for three and one half years,and at the end of that time the saints that remain are gathered unto Christ.
Then the great tribulation is over,for the great tribulation is for the saints,not the world,and then the wrath of God poured on the people that followed the beast kingdom,leading up to the battle of Armageddon,where the plague God gives them is their flesh shall melt off their bones,which the wrath of God is not tribulation,but punishment for following the beast kingdom.
The great tribulation is for only three and one half years,and it is only for the saints.