Rabbi Berach Steinfeld
The Torah tells us in Vayikra, Perek Chof Zayin, posuk lamed daled that these are the mitzvos that Moshe
The Rambam in Perek Gimel in Hilchos Melachim, halacha ches paskens that if a Jew rebels against a Jewish king, the king has a right to kill him. We learn this from the posuk in Yehoshua, Perek Alef, posuk yud ches that says that any person who will rebel against Yehoshua shall be put to death. If you analyze the posuk it sounds like this is something that Klal Yisroel were mekabel upon themselves. In the Sefer Hamitzvos of the Rambam, mitzvah kuf ayin gimmel the Rambam says that if one is oveir on the command of the king, it is proper and permissible for the king to kill him. This sounds like it is not a command, but rather something the Yiddenaccepted upon themselves.
In the Shailos U’tshuvos Chasam Sofer, siman resh ches the Chasam Sofer says that we do not find it stated anywhere in all five sefarim of the Torah that a king is allowed to kill someone. Yet Yehoshua uses the words, “Al pi Hashem” when he writes that someone rebelling can be put to death. How could a navi make a new mitzvah? The Chasam Sofer ends off that this question requires a lot of deep thought, but he does provide an answer.
One could try to answer the question by saying that we find many times in Shas that the Gemara asks the question how could a navi be mechadesh something? The Gemara answers in many places that there were many halachos and different things that the Yidden forgot during the time they mourned for Moshe Rabbeinu and the navi re-established it. We find this concerning a lot of hilchos kehuna that we learn from the Sefer Yechezkel. We say that it was forgotten and then re-established by the Navi Yechezkel. Here too, we might say that it was forgotten and re-established by Yehoshua.
We could argue and differentiate that anything told by the navi in a commanding way is probably a mitzvah in the Torah that was forgotten and the navi re-established it. In the event that the navi says it in story form and not in a commanding form, then we won’t say that it was forgotten and re-established. This is the reason why the Chasam Sofer asked the question since what Yehoshua said was not a command, but rather something said in story form that the Jews accepted upon themselves about a king being able to kill someone who rebels. This is why the question applies; how could the Navi be mechadesh
We could answer that the fact that the king could kill someone is not a new mitzvah or an addition to a mitzvah since in that case it would be forbidden for a navito make a new mitzvah. In the case where the Torah says something but does not explain it, then the navi is able to explain what the Torah meant. An example of this would be that we find that the Torah uses the word “veshinantam” (it should be clear) regarding the mitzvah of learning Torah. The Navi Yehoshua explains this as meaning “vehagisa,” you shall be busy with it day and night. This is despite the fact that Min Hatorah if one just says Shema in the morning and night one is yotzei the mitzvah of Talmu
May we be zocheh to see the “Melech Moshiach” speedily in our days.