WASHINGTON—Ukraine’s weekend attacks against military airfields deep inside Russia signal the long-stalemated war is entering a perilous phase, with both sides seemingly intent on escalation and prospects for a U.S.-brokered peace deal receding.

       Only a day after the Ukrainian drones dealt a blow to Russia’s bomber fleet in a brazen attack that stunned Moscow, the two sides met for a second round of talks in Istanbul after President Trump declared again that he wanted a quick deal to halt the fighting.

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