What promise did Ellen make to Annemarie?

From the text:

One by one the Rosens turned and hugged Annemarie silently. Ellen came to her last; the two girls held each other.

"I'll come back someday," Ellen whispered fiercely. "I promise."

"I know you will," Annemarie whispered back, holding her friend tightly.

What promise did Ellen make to Annemarie? (Chapter 12)

The promise was that Ellen will be back someday.

What does Annie Marrie mean on page 98 when she says " it was harder for the ones who were waiting... less in danger,perhaps,but more fear