When a neighboring nation witnesses the downfall of the Israelites, their reaction goes beyond silent hostility. The Ammonites express a deep hatred through an intense, physical display of malicious joy over the ruin of the Israelites, prompting a severe response from God. His anger is directed precisely at this outward, physical celebration of another's tragedy.
They celebrate the destruction by enthusiastically clapping their hands and stomping their feet in a violent dance. The stomping is so forceful that it mimics the act of flattening the earth, similar to beating out metal [מצודת דוד, מצודת ציון]. Interestingly, these exact physical gestures were commanded earlier as expressions of profound grief and mourning. Commentators agree that human actions can serve entirely opposite emotional purposes depending on the context. This is much like how ancient flutes were played both to celebrate weddings and to mourn at funerals [רש"י, מצודת דוד, חומת אנך].
Alongside this physical outburst, there is a profound psychological element to their celebration. The primary approach among commentators is that their inner joy is filled with deep scorn, mockery, and contempt for the land itself [רד"ק, מצודת דוד, מצודת ציון, אברבנאל]. Other perspectives offer different shades of this emotion. One approach describes it as a bursting, overflowing joy [רש"י]. Another connects their attitude to a whip, suggesting a stinging, cruel joy intentionally designed to mentally strike and hurt the Israelites through mockery [מלבי"ם].
The intensity of this joy stems from a few different sources. On a practical and political level, the Ammonites celebrated because they used the opportunity of the destruction to actively invade and settle on the land [מלבי"ם]. Looking deeper into the symbolism of their actions, the celebration targets three distinct tragedies that struck the Israelites. The clapping of hands represents joy over the desecration of the Temple. The stomping of feet symbolizes celebration over the exile and loss of the people. Finally, their deep inner scorn is directed at the sheer desolation of the land itself [אברבנאל].