OK, here goes....
What did they actually believe before? (my emphasis)
Watchtower 1 March 2001, page 15, paragraph 17
In 1918 they went into spiritual captivity when leading officers of the Watch Tower Society were imprisoned. The organized preaching work virtually ceased. Then, in 1919, Jehovah restored them and reinvigorated them with his spirit, whereupon they set out to fulfill the commission to preach the good news in all the inhabited earth.
Isaiah Book Vol 2 (2001) page 181, paragraph 4
During the first world war, however, her representatives on earth — anointed Christians — unwittingly broke Jehovah’s law because they did not properly understand true Christian neutrality. Losing divine favor, they came into spiritual captivity to “Babylon the Great,” the world empire of false religion. (Revelation 17:5) Their condition of being in slavery climaxed in June 1918 when eight staff members of the Watch Tower Society were imprisoned on false charges, including conspiracy. At that point the organized preaching of the good news virtually stopped. In 1919, however, a clarion call to spiritual wakefulness was sounded forth. Anointed Christians began to separate themselves more completely from the moral and spiritual uncleanness of Babylon the Great. They rose from the dust of captivity, and “Jerusalem above” came to have the splendor of a “holy city” where spiritual uncleanness is not allowed.
Isaiah Book Vol 2 (2001), page 241, paragraph 16
The climax came in 1918 when the public preaching work virtually stopped and responsible officers of the Watch Tower Society were imprisoned on false charges of sedition. In this way, Jehovah’s modern-day servants went into a spiritual captivity, reminiscent of the physical captivity of the ancient Jews. Great reproach hung over them.
Note capacity is tied to the infamous imprisonment of those bethel members from June 1918 to 1919 - and it was: "At that point the organized preaching of the good news virtually stopped." (what does the Study Watchtower for Sunday 29 Jan - paragaph 7 - state about the preaching work during that time, June 1918 to 1919 - no not during 1914, or 1916, that wasn't the time of spiritual capacity)
What do they believe now?
Study Watchtower March 2016, pages 29 to 31
Questions From Readers: When were God’s people held captive by Babylon the Great?
That spiritual captivity lasted from the second century C.E. to 1919.
NOTE - in the QFR answer below, that infamous imprisonment of those bethel members has been completely dropped from the explaination and isn't mentioned at all... hmmmm....