Ukrainian troops fell back to defensive lines amid one of Russia's largest offensives since the beginning of the full-scale invasion more than two years ago. President Volodymyr Zelenskyy has postponed all foreign travel, and U.S. Secretary of State Blinken announced new weapons deliveries while visiting Ukraine. But as Nick Schifrin reports, in Kharkiv, the front is fragile. PBS NewsHour is supported by - https://www.pbs.org/newshour/about/funders