In the seventh aliyah, the Jews journey further and run out of water and complain to Moshe. Moshe relays this to G-d and G-d tells him to smite a certain rock with his staff so that water will come out of it for the people to drink.
Then the nation of Amalak comes and fights with Israel. They are always remembered as the first ones to attack the Jews, thereby breaking the ice, so to speak, showing other nations their example of making war with the Jews. Our sages tell us that the wicked Haman descended from Amalak.
Another aspect of this war of Amalak is that they attacked on the Jews' way to Mt. Sinai to receive the Torah, and therefore attempted to dampen their enthusiasm for receiving the Torah by causing trouble on the way.
Chassidus teaches that whenever our enthusiasm for a mitzvah is dampened, it's our personal internal Amalak doing it to us, and we are enjoined to blot it out. It is interesting to note that the word for "doubt" in Hebrew, is "sofek", which has the same gematria (numerical value) as Amalak (240). So whenever we have a doubt that slows or prevents a mitzvah, that is Amalak slowing or preventing that mitzvah.