During the war of ח"תש in Yerushalayim, the Brisker Rav zt'l heard two people speaking. They were saying, "The army ('the צבא') won the battle at that location... and the army won the battle over there..." The Brisker Rav became frightened when he heard their words, and turned to them and said, "Do you think the צבא won the wars? Not at all! Reb Velvel Chechik and Reb Shlomo Bloch are from the tzaddikim of Yerushalayim. They arranged minyanim of tefillah around the clock, 24 hours a day. Those tefillos won the war!
The men responded, "So why are there people who were wounded in this war?" Their question was that if the war was won with tefillah, there should be no casualties. The Brisker Rav told them a story that happened with his grandfather, the Beis HaLevi. The Beis HaLevi's son was engaged to be married, and the mechutanim promised a large dowry, in honor of the chasunah. The Beis HaLevi said to his son, "Don't feel proud that you are receiving a large dowry, because it isn’t in your merit. They are giving the large dowry because you are my son." His son asked, "If they are giving the money because of you, they should give ten times more. Since they aren't giving so much money, it proves that the dowry is for me." The Beis HaLevi replied, "The truth is, they would have given a lot more. Only, you are also part of the story here, and this is the reason for the smaller gift."
Using this story, the Brisker Rav said to the two men, "You are correct. With tefillah, there should have been only success in the war, and no one should have been harmed. Only, the צבא was also part of the equation, and that is the reason there were some casualties."
After the war against Amalek, it states (Shemos 17:15) נִסִּי 'ה שְׁמוֹ וַיִּקְרָא ַמ ִזְבֵּח ֶׁהמֹש וַיִּבֶן, "Moshe built a mizbeiach, and he named it 'Hashem is my miracle.'" The Ksav Sofer asks why Moshe chose this specific miracle to be memorialized by naming the mizbeiach after the miracle. After the nissim in Mitzrayim and at kriyas Yam Suf, Moshe didn't build a mizbeiach and name it נסי 'ה, "Hashem is my miracle." Why did he do so specifically after milchemes Amalek?
The Beis HaLevi answers that everyone knew that the miracles in Mitzrayim and kriyas Yam Suf were from Hashem. But after the war with Amalek, there will be those who think it was they who fought and won because they had a stronger army. They will say that this is the way of war, one nation wins and the other loses. Therefore, after this miracle, Moshe built a mizbeiach and named it נסי 'ה, "Hashem is my miracle", so it should be clear and known that the miracle and the success in the war was from Hashem, and not due to their strength.
