BRUSSELS (Reuters) - Almost a year after U.S. President Donald Trump inaugurated NATO’s new billion-dollar headquarters with fanfare at a special summit, alliance officials are now finally moving in, leaving their prefab 1960s base that leaked during rainstorms.

Two decades of planning, construction and an additional year-long delay caused by technology problems now appear to be behind the Western alliance.

Its international staff and 29 embassies should be rehoused by mid-June, ready for a NATO defense ministers’ meeting and then a two-day NATO summit of alliance leaders in July, which Trump is expected to attend, officials say.

But within the shiny glass and steel interlocking buildings, NATO allies will face familiar problems, including how to handle a newly assertive Russia and manage the collective defense of Europe.

The move was originally due to take place soon after the May 25, 2017 visit by the U.S. president and other Western leaders. Trump, a former property developer, praised the 1.17-billion-euro ($1.45 billion) building as beautiful but said he had refused to ask how much it had cost.

NATO Deputy Secretary General Rose Gottemoeller said the move was about a year behind schedule.

“I don’t think that’s bad for an enormous building project,” she told reporters but conceded that rainwater dripped into her office in NATO’s current headquarters during a recent heavy winter storm.

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The delay has stemmed in part from an ambitious plan for a central IT system to act as a brain to run the building. That was complicated by contractor Lockheed Martin Corp’s deal to sell its IT services division in early 2016.

Leidos Holdings Inc completed the job but “the handoff from one company to another was a factor,” Gottemoeller said. The building has 60,000 sensors helping control everything from the temperature of meeting rooms to secure-area doors.

The 4,200 staff will work in a total office space similar in area to the United Nations headquarters in New York. Except that to meet Belgian building requirements, the NATO HQ is only 32 meters (105 ft) tall at its highest point.

Seen by some as a Cold War relic until Russia’s 2014 annexation of Crimea, NATO has gained new relevance as the West confronts Russia, cyber warfare, militant attacks in Europe and seeks to defend against the threat of ballistic missiles from Iran to North Korea.

“MAGIC CARPET”

With glazing equivalent to 10 football pitches, sleek, airport terminal-like halls and the expanded, amphitheatre-like North Atlantic Council decision-making chamber, many staff currently housed in the 1967 headquarters across the street in Brussels have been eager to move for months.

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Last year at the May summit, with jets flying overhead, NATO used the new building to project an image of power and renewal, avoiding too much talk about its unfinished status.

That has lead to some confusion among non-NATO diplomats and residents of Brussels about which of the two buildings - the 1960s prefab, or the glass palace - were working.

Until June, both are. NATO officials shuttle between the two buildings, as delegations shift boxes and archives, on a special bus service dubbed “the magic carpet” by officials.

NATO decided it needed a new home in 1999 because its headquarters in Brussels were only meant to be temporary. They were built in a hurry when the alliance was forced to leave Paris after France’s withdrawal from the North Atlantic Treaty Organisation’s military command in 1966.

In a throw-back to NATO’s origins in the Cold War, the new headquarters features two concrete panels made up of masonry from the Berlin Wall.

It also displays a piece of the wreckage from the 107th floor of one of the Twin Towers in New York, destroyed during the Sept. 11, 2001 attacks on the United States -- the only time the alliance has activated its collective defense clause.