Thousands of miles from the Persian Gulf, economic and political shrapnel from the Iran war has left longtime American allies nervous about the staying power and stability of their partnerships with the United States. The most recent spillover from the war has come in Europe: last week’s sudden decision by U.S. President Donald Trump to withdraw 5,000 American troops from Germany. But it’s also been buffeting allies in the Asia Pacific, an area of the world Mr. Trump has made a primary foreign policy priority, ahead of his scheduled visit to Beijing later this month for talks with Chinese leader Xi Jinping. Why We Wrote This Ahead of President Donald Trump’s expected visit in Beijing with Chinese leader Xi Jinping, longtime American allies in Europe and Asia remain on edge about the strength of their partnership with the United States. Even before the war, Mr. Trump’s use of tariffs to secure U.S.…