Renia Spiegel's (left) war diary is strikingly similar to that of Anne Frank.

As the Nazis strengthened their grip on Europe in 1939, Renia Spiegel, a 14-year-old Jewish girl in Poland, turned to a new companion. "Today, my dear Diary, is the beginning of our deep friendship," she wrote on Jan. 31 that year. For more than three years, Renia documented her life in more than 650 handwritten and densely filled pages.

"You won't betray me," she wrote.

But someone else did.

On July 30, 1942, Renia, then 18, was summarily executed by the Nazis after they discovered her hideout in the city of Przemysl in southeastern Poland.

More than half a century later, her translated diary - strikingly similar in some ways to that of Anne Frank but long kept locked in a vault - is finally set to be published in the United States. The scheduled release Tuesday comes at a critical time, as nationalism and right-wing populism have drawn comparisons with the 1930s - including in the United States, where Renia's sister Elizabeth Bellak fled with her mother after World War II.

"I'm worried again," said Bellak, 88, speaking from a hotel in Warsaw this week, where she was due to attend the screening of an accompanying video documentary by Polish American filmmaker Tomasz Magierski. She expressed hope that her sister's story would help "remind young and old" of the horrors of that time.

Bellak still has not found the strength to read the full diary her sister wrote up to her final days.

"It's too emotional, too painful for me," she said. Renia, as the older sister, "was like a surrogate mother to me" and a "wonderful, wonderful person."

"Whenever her heart was broken, she wrote a poem," Bellak said.

Renia's diary indicates how much she cared for her sister, a child movie star in the late 1930s Poland, despite occasional discontent over what she saw as her younger sibling's ability "to steal the show."

The diary traces Renia's path from a teenager worried about the looming threat of Nazism to a victim of the Nazis' occupation of Poland and their declared mission to exterminate the Jewish people.

Renia Spiegel's sister (centre), who she dearly loved, said she never had the courage to read the full diary.

Weeks before the invasion of Poland, Renia already appeared to sense growing danger. "Mama's very worried about me. Oh! I'm so unhappy," she wrote in March 1939. Her mother spent long stretches in the capital, Warsaw, in 1939 and following years, hoping to promote her younger daughter as a movie actress.

Renia and her sister stayed behind in southeastern Poland, where Renia continued to document her growing anxiety. But she also wrote about falling in love and trying to go about normal life. Later that year - still separated from her mother - Renia wrote of an intensified sense of urgency: "Przemysl was attacked. We had to flee. The three of us escaped: me, [Elizabeth] and Grandpa. . . . Granny stayed behind."

Weeks later: "Holy God, please give me an easy death." Within the subsequent three years, in hundreds of detailed entries, Renia documented their family's transfer into a ghetto, along with thousands of other Jews who were given only 24 hours to move. The Nazis were readying themselves for the later stages of the Holocaust, and their grip on the country was intensifying.

As the Nazis were preparing to transfer thousands of Jews to a death camp, Renia's boyfriend, Zygmunt Schwarzer, arranged a desperate attempt to rescue the sisters - the two were separated from one another, and from their grandparents.

"I'd give anything to remember our last words to each other. I'd give anything to know that I told her how much I loved her," Bellak wrote in the book's afterword about the lost memories of her farewell with Renia.

Separating from her grandmother was painful, as well. "My granny, whom I loved so much, turned away, raising her hands to her face," Bellak recalled.

Soon after their goodbye, the Nazis uncovered Renia's hideout, and she was summarily executed. The fate of her grandparents is unknown, but Bellak believes they were killed and buried in a mass grave.

She and her mother managed to escape to Austria with the help of a German officer who had fallen in love with the mother, a fluent German speaker. After the war ended, both emigrated to the United States, where they were later joined by Renia's boyfriend.

"I have something for you," Bellak recalled Schwarzer saying at the time.

"It was Renia's diary, all seven hundred pages of it. My mom and I broke down in tears," Bellak wrote, recalling the encounter in the early 1950s. Throughout their time together, she had not known that her sister was writing a diary.

After the death of Renia and of Schwarzer's own parents - who had been hiding in the same place - the boyfriend finished the diary with these words: "Fate decided to take my dearest ones away from me. My life is over. All I can hear are shots, shots . . . shots."

But despite the haunting details - or perhaps because of them - Renia's diary lay virtually untouched in a vault in New York City for more than half a century.

"I can't explain why I was allowed to live, and that's why I've tried for so long to turn my mind away from it," Bellak recalled in the afterword of her sister's diary.

In New York, Bellak went on to attend Columbia University and later married.

When her own children started asking about their family's story, she gradually began opening up.

"What was in that book about the aunt?" Bellak recalled her daughter asking repeatedly. "That's how it started."

Both filmmaker Tomasz Magierski and Bellak's daughter, Alexandra Bellak, played a crucial role in the release of the diary, initially in Polish and now in other languages.

"My mother was reluctant to talk about it because it unearthed painful memories," the daughter told The Washington Post. "So, I thought, well, there's this elusive diary, sitting in the vault." The more pages were translated, the more obvious the diary's literary quality and historical relevance became to her.

"Writing gave her freedom to express herself," even as "evil and hate all around" mounted, Alexandra Bellak said of Renia.

"It's almost fortuitous that the diary is being published now . . . because the same signs are showing their ugly heads again," she said. "It's more relevant than ever."

This week, Elizabeth Bellak returned to Hotel Europejski in Warsaw - where she and her mother had sought refuge before they eventually fled abroad.

"I survived the war, I survived life, and here I am again after 75 years" - this time to call attention to the story of her family and its contemporary message, Bellak said by phone from the hotel this week.

Those who will read about Renia's story, Bellak said, "should try to help the world not to become what it was then."