Mia Caballeros of the Andersonville neighborhood planned an epic European vacation to celebrate a milestone birthday and her 10th wedding anniversary — first visiting friends in the Netherlands and then onto Spain.

       Yet she spent the week before the trip in a panic because the passport she applied for in May still hadn’t arrived in early July, amid an unprecedented backup in U.S. passport applications that has upended international travel this summer.

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       Caballeros even paid extra fees for expedited processing and delivery to ensure the passport would come in time. Then she spent hours on the phone and online scrambling to try and snag an in-person appointment at a federal government passport agency, but the only one available was the day of her flight.