After David ends his period of מלחמות רשות and creates his kingdom of (שמואל ב ח:טו) עשה משפט וצדקה לכל עמו, he sends a delegation to Amon:
Now, this is strikingly different from the way he treated Amon’s sister state, Moav:
There is no hint in תנ״ך what the king of Amon might have done, that David would say כאשר עשה אביו עמדי חסד. Even the most stringent בעלי פשט have to go back to the Aggadah:
And then David’s family disappears, except for one brother who becomes the נשיא of יהודה:
But Amon’s response is shocking:
As an aside, David has them wait in ירחו because it’s right over the Jordan, and because no one lived there:
What went wrong in David’s overture to Amon? חז״ל read תנ״ך as ספרי נבואה, so there must be a message in the text’s choice of what incidents to relate. There is an inherent problem in trusting Amon:
The pasuk of אַל תְּהִי צַדִּיק הַרְבֵּה is the same one חז״ל use to describe Saul who had pity on the king of Amalek:
The Kesef Mishna proposes a reason that David might have felt that he was justified in his overture to Amon:
The משבצות זהב makes the same point, noting that the pasuk says וישלח דוד לנחמו ביד עבדיו אל אביו, not דוד המלך ביד שרי המלוכה, and they went לנחמו אל אביו, not as king of Amon but as a son in mourning.
But the truth is that the king represents the people as a whole. Nothing David does now represents himself as an individual. Whatever good Amon did to him personally does not outweigh the utter evil that Amon not so long before had committed against Israel:
And here Amon proves the point by humiliating David despite his best intentions.
How does David respond to this provocation? We don’t see in the text (the next psukim are about Amon’s response to how they think David will respond), but there is a chapter of תהילים that may be relevant:
Artscroll combines these as “Only for G-d does my soul wait silently”.
The phrase לא אמוט רבה seems grammatically off; it literally means “I will not falter large”. The commentators have to add words:
Even so, it seems to be damning by faint praise: ה׳ will only let me slip a little? Sforno reads רבה as an apostrophe. This perek is addressed to עמון and its capital רבה, or רבת בני עמון:
תְּרָצְּחוּ could be read as “may you all be murdered”:
But that doesn’t seem to fit the theme of trusting in ה׳. Artscroll cites Rashbam and Ibn Yachya (neither of which I can find) that this is David addressing his enemies, “You all plan to murder me!”. His enemies are subtle, setting up the situation so they cannot be blamed:
And more that that, they would arrange for their victim to be liable for the damage:
אך משאתו יעצו להדיח means “only because of his—the איש of פסוק ד—elevated stature do they plot to topple him”.
And Sforno continues to read all this as referring to the Amonites:
This stich expresses the same faith in ה׳, but here it’s addressed to the people: בטחו בו בכל עת, עם rather than the רשעים.
Then David concludes that there is no point in worrying about their evil plans; it’s all הבל. That’s usually translated as “vanity” but that word has a different meaning in modern English. הבל means “breath, puff of air”.
אחת דבר אלקים שתים זו שמעתי in the פשט is simply emphatic: “ה׳ has said this not once but many times”.
חז״ל saw in this pasuk the idea that Torah has multiple levels of meaning; one word of G-d should be heard in multiple ways:
But the meaning in context is that ה׳ has said over and over again that He will mete out the appropriate punishment; אתה תשלם לאיש כמעשהו. There is no reason for David to seek revenge against Amon. He thinks he has learned the lessons of the wars with Moav and Edom. But life is never that easy.