The battles fought between the Jews and their enemies, which took place on Adar 13 throughout the Persian empire (see "Today in Jewish History" for that date), continued for two days -- Adar 13 and 14 -- in the capital city of Shushan, where there were a greater number of Jew haters. Thus the victory celebrations in Shushan were held on the 15th of Adar, and the observance of the festival of Purim was instituted for that day in Shushan and all walled cities. (See Laws and Customs below).
On this date, in the year following the Holy Temple’s destruction, G‑d tells Ezekiel to take up a lamentation for Pharaoh, king of Egypt, and for the Jewish nation’s other enemies, foretelling their ultimate downfall.
Read the prophecy here: Ezekiel ch. 32
In cities that are surrounded by a wall dating from the days of Joshua (13th century BCE) -- a prominent example is the city of Jerusalem -- the festival of Purim is observed on the 15th of Adar (instead of the 14th), in commemoration of the fact that in the ancient walled city of Shushan, the first Purim was celebrated on this day (see "Today in Jewish History").
All over the world, Tachanun is omitted due to the special joy of the day.
(For an overview of the Purim observances and links to more information, see "Laws and Customs" for Adar 14.)
Rabbi Abba son of Kahana said, “David’s generation were all righteous, but because there were informers among them, they fell in battle...Ahab’s generation were idolaters, but because there were no informers, they went to battle and were victorious.”
Jerusalem Talmud, Peah 1:1.
Jerusalem is the city that joins all the tribes together as one. When the Jewish people come together as one, nothing can destroy Jerusalem. And only when there is love and caring between them is the destruction and our exile healed.
When a father sees his children working together with love, he is prepared to give them anything.
Yes, love for one another means also to care enough to encourage the other to do mitzvahs and learn Torah.
But first comes the love.