Time after Time by Josie Kerr

Christine Dearborn met her soulmate in high school. They eloped shortly before he went to boot camp, allowing her to escape from her austere, loveless childhood home. But being a military wife came with its own challenges, and Christine fought her own enemies at home even as her beloved battled adversaries overseas.
William Cashton had always been a guardian. When he was eighteen, he promised to protect his wife and his country. Fighting a foreign enemy turned out to be the easy part, and the war at home has more devastating casualties than Cash ever imagined.
Five years after she let Cash walk away, Christine gets a phone call that changes everything. Can Christine convince Cash to try to salvage their marriage, or will their emotional wounds be fatal?