By Alexandra Hudson BERLIN (Reuters) - NATO is at its most cohesive yet, its new commander said, but must ensure that spending cuts do not erode the power of an alliance facing tasks as diverse as a new training mission in Afghanistan and protecting Turkey's border with Syria. Many European countries have slashed defense spending since the 2008 financial crisis. Apart from the United States, Greece, Britain and Estonia, no member has met NATO's goal of spending 2 percent of economic output on defense. U.S. ...