"Special relationships" are often volatile in international politics as they are in the personal sphere. After a long period of harmonious partnership, one couple may decide on a trial separation, while another may choose to renew their vows. This appears to be what is happening now in Anglo-American and U.S.-Japan relations respectively.
Britain's May 7 general election is widely expected to usher in a period of weak government that empowers minority parties and leads to further arguments about Scottish independence, a British "exit" from the European Union and the desirability of Britain's costly nuclear deterrent. All this is anathema to the U.S., which is already displeased with U.K. Prime Minister David Cameron's decision to sign up to the China-led Asian Infrastructure Investment Bank and his reluctance to get involved in the Ukraine crisis.