ANKARA, Turkey, July 7 (Reuters) - President Donald Trump said on Tuesday Greenland should be controlled by the United States, not Denmark, reaffirming a stance that has caused tensions among NATO allies just as leaders of the alliance gathered for a summit in Turkey.Trump's assertions that the U.S. must acquire or control Greenland, a semi-autonomous Danish territory, have long strained relations between Washington and Copenhagen — both founding NATO members — and more broadly U.S. ties with Europe. The issue has since moved to a diplomatic track.