The 187th mitzvah is that we are commanded to kill and destroy the seven nations [of Canaan]1 because they are the prime worshippers and original source of idolatry.

The source of this commandment is G‑d's statement2 (exalted be He), "You must wipe them out completely." [Scripture3] explains the reason for this commandment is to keep us from learning from their heresy. Many verses encourage and urge that they be killed, and waging war against them is a milchemes mitzvah [mandatory war].

Since these seven nations no longer exist4 a person could think that this commandment is not noheg l'doros [for all generations5]. But only someone who does not understand the concept of noheg l'doros would think such a thing. A command that can be fulfilled without being limited to a certain time is considered noheg l'doros, because if the act would become possible in any generation, the mitzvah would apply. When G‑d will totally destroy the descendants of Amalek and remove them for all time — as will be speedily in our days, as G‑d (exalted be He) promised,6 "I will wipe out the memory of Amalek" — will we say that the mitzvah to wipe out the memory of Amalek7 was not noheg l'doros? This is not true, for in any generation when one finds a descendant of Amalek, he must be killed. The same applies to this mitzvah of killing all descendants of the seven nations, which is a milchemes mitzvah. In every generation we are required to uproot them and search after them down to the last individual. We did this until King David destroyed them completely, with the survivors being scattered and assimilated among the nations until they disappeared.

But although they no longer exist, the mitzvah to kill them is still considered noheg l'doros, just as the mitzvah to wage war against Amalek is considered noheg l'doros even after their destruction. This is because it is not dependant on a certain time or place, such as in Egypt8 or in the desert.9 The mitzvah is dependant solely upon the object of the mitzvah: whenever they are found, the mitzvah must be fulfilled.

The general rule is that you must understand and contemplate upon the difference between the commandment itself10 and this that the commandment deals with.11 There are mitzvos where the object of the commandment has ceased to exist in a certain generation,12 but this does not render the mitzvah not noheg l'doros, since the commandment itself applies forever.

For a commandment to be considered not noheg l'doros, the opposite would be true. The specific object in the specific state does exist; but the obligation to perform the specific act or follow the certain law only applies at a certain time. Today, even though the object exists, the commandment does not. An example of this would be an elderly Levite, who was not allowed to serve [in the Mishkan] in the desert, but is allowed today, as we explained in the proper place.13 Be sure you understand this and keep it in mind.