The Kremlin said Thursday it regretted the expiration of the last remaining nuclear arms pact between Russia and the United States, while President Trump declared he was against keeping its limits and wants a better deal. The New START treaty’s termination left no caps on the two largest atomic arsenals for the first time in more than a half-century, fueling fears of an unconstrained nuclear arms race. Russian President Vladimir Putin last year declared his readiness to stick to the treaty’s limits for another year if Washington followed suit, but Mr. Trump has ignored the offer and argued that he...