Parashat Ha’azinu relates that Hashem told Moshe "b’etzem hayom hazeh" (in the midst of this day) to go to Har Nevo and die (Devarim 32:48). The Sifrei points out that this is one of three places that the Torah writes (in a narrative setting) b’etzem hayom hazeh. The others are Noach entering the ark and Bnei Yisrael leaving Egypt, both during the day. The message in all, according to Chazal, is that Hashem was demonstrating that He would make the history-altering event happen before the eyes of those who opposed it and show them that they were incapable of stopping it.
Rav Neventzal (Sichot L’Yom Hakippurim, pp. 95-104) asks a simple but insightful question. Hashem did challenge Noach’s neighbors to prevent Noach from entering the ark and the Egyptians from allowing Bnei Yisrael from leaving. Each was naturally capable of preventing the event. However, why would Bnei Yisrael think that they could prevent Moshe from dying?
One could answer based on the Midrash Tanchuma (Vaetchanan 6) that Hashem offered Moshe that he could enter the Land, but the price was that Bnei Yisrael would have to be destroyed, an offer Moshe rejected. Perhaps Bnei Yisrael would have taken a drastic step of agreeing to have a plague or the like to take responsibility for its shortcomings, thus forcing Hashem to allow Moshe to live.
In any case, Rav Neventzal cites a different answer in the name of Rav Chaim Shmulevitz. Had Hashem allowed them to do so, Bnei Yisrael could have prayed in an extremely sincere, impassioned manner for Moshe’s survival, which would have kav’yachol forced Hashem to accept their prayers. We find a similar concept in the gemara in Ketubot (104a) regarding the death of Rebbi (Rabbi Yehuda Hanasi). He was suffering greatly as he was on the verge of death, as the righteous of his generation prayed beneath his home for his survival. Seeing his situation, his servant prayed that the heavenly beings who wanted Rebbi should overcome the people who wanted him to live, but the people were praying too strongly, and he thus did not die. The servant threw a jug near the crowd, with the explosion silencing the prayers for long enough for Rebbi to die.
We see from these sources how exceptionally strong the power of tefilla is. Of course, not every prayer is intense enough to be a formidable force for Hashem to reckon with. As Rav Neventzal points out briefly (and we, even more briefly) not every wish should be prayed for and not always does one get the response he desires. But every proper prayer is noticed and leaves some kind of mark in the Heavens and the earth.
When we say that "repentance, prayer, and charity can remove the harm of the decree" these words are literal and powerful beyond our natural expectations. Let us take them seriously and make the most of the powerful tools we were granted.