SteveVH,
There is an added thought for your question about the following:
D&C 84:
2 Yea, the word of the Lord concerning his church, established in the last days for the restoration of his people, as he has spoken by the mouth of his prophets, and for the gathering of his saints to stand upon Mount Zion, which shall be the city of New Jerusalem.
3 Which city shall be built, beginning at the temple lot, which is appointed by the finger of the Lord, in the western boundaries of the State of Missouri, and dedicated by the hand of Joseph Smith, Jun., and others with whom the Lord was well pleased.
4 Verily this is the word of the Lord, that the city New Jerusalem shall be built by the gathering of the saints, beginning at this place, even the place of the temple, which temple shall be reared in this generation.
The added thought (besides the quote by Zerinus that also makes sense) is that you identify the clauses of the “prophecy and commandment”, (as you might wish to call it either way). Clause #1 is that the New Jerusalem shall be “built by the gathering of the saints, beginning at this place” which was at the place where they were gathered and where they began the work of building the New Jerusalem and gathering the saints (you see, the gathering of the saints begins the work of building the New Jerusalem, since the New Jerusalem is the fulfillment of Zion and Zion begins to be built wherever the saints gather in a place of holiness and peace with a temple in their midst).
Clause #2 is the phrase “which temple shall be built in this generation”, and that temple was built indeed, in Nauvoo, IL, where the saints received their endowment with power from on High before their journey west to escape those who had robbed them. God had commanded them to build a temple, and they did despite the opposition of the forces of destruction in the world–the forces which always oppose the building of Zion wherever the saints go to with their might and build it. They find a way, and they did find a way, and indeed the New Jerusalem will be built during the latter days in America, the land of a latter-day Zion (among others in the world). But it is a long process. Not a problem–an opportunity, day by day and year by year. The building up of places of Zion in the world fulfill the prophecies of Isaiah.