NEW YORK — Europe appears on the brink of another recession. Islamic militants have seized Iraqi territory. Russian troops have massed on the Ukraine border, and the resulting sanctions are disrupting trade. An Ebola outbreak in Africa and Israel’s war in Gaza are contributing to the gloom.It’s been a grim summer in much of the world. U.S. stocks sagged Friday on reports that Ukrainian troops attacked Russian military vehicles that had crossed the border. But investors in the United States have been shrugging off most of the bad news — so far at least.A big reason is that five years after the Great Recession officially ended, the U.S. economy is showing a strength and durability that other major nations can only envy. Thanks in part to the Federal Reserve’s ultra-low interest rates, employers have ramped up hiring, factories have boosted production and businesses have been making money. All of this has cushioned the U.S. economy from the economic damage being suffered abroad. And investors have responded by keeping U.S. stocks near all-time highs.China, the world’s second-biggest economy, is struggling to contain the fallout from a runaway lending and investment boom that’s powered its growth since before the 2008 financial crisis. The economies of Japan and Germany, the world’s third- and fourth-largest, shrank in the spring. So did Italy’s.