KYIV, Ukraine — A powerful attack on a Russian air base in occupied Crimea was the work of Ukrainian special forces, a Ukrainian government official told The Washington Post on Wednesday, suggesting an increasingly important role for covert forces operating deep behind enemy lines as the country expands efforts to expel Russian troops. The official, who spoke on the condition of anonymity because they were not authorized to speak publicly on the matter, did not disclose details of how Tuesday’s attack was carried out. The attack marks a significant escalation in the nearly six-month-old war, demonstrating a new ability by...