The UN was once the League of Nations and at that time, 1919, Christendom hailed the League as God's means of bringing peace to the earth...putting it in the place the Messianic kingdom should be in referring to the 1st century when the Roman army took the place of the temple in Jerusalem (disgusting thing). The WTS has changed the timing of that to a future one.
What will signal the start of the great tribulation? Jesus foretold: “When you catch sight of the disgusting thing that causes desolation, as spoken of through Daniel the prophet, standing in a holy place, (let the reader use discernment,) then let those in Judea begin fleeing to the mountains.” (Matt. 24:15, 16) In the first fulfillment, the “standing in a holy place” occurred in 66 C.E. when the Roman army (“the disgusting thing”) attacked Jerusalem and its temple (a place holy in the eyes of the Jews). In the larger fulfillment, the “standing” will occur when the United Nations (the modern-day “disgusting thing”) attacks Christendom (which is holy in the eyes of nominal Christians) and the rest of Babylon the Great. The same attack is described at Revelation 17:16-18. That event will be the beginning of the great tribulation.