Save "Shelach -- "But"
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Shelach -- "But"
(כה) וַיָּשֻׁ֖בוּ מִתּ֣וּר הָאָ֑רֶץ מִקֵּ֖ץ אַרְבָּעִ֥ים יֽוֹם׃ (כו) וַיֵּלְכ֡וּ וַיָּבֹאוּ֩ אֶל־מֹשֶׁ֨ה וְאֶֽל־אַהֲרֹ֜ן וְאֶל־כָּל־עֲדַ֧ת בְּנֵֽי־יִשְׂרָאֵ֛ל אֶל־מִדְבַּ֥ר פָּארָ֖ן קָדֵ֑שָׁה וַיָּשִׁ֨יבוּ אוֹתָ֤ם דָּבָר֙ וְאֶת־כָּל־הָ֣עֵדָ֔ה וַיַּרְא֖וּם אֶת־פְּרִ֥י הָאָֽרֶץ׃ (כז) וַיְסַפְּרוּ־לוֹ֙ וַיֹּ֣אמְר֔וּ בָּ֕אנוּ אֶל־הָאָ֖רֶץ אֲשֶׁ֣ר שְׁלַחְתָּ֑נוּ וְ֠גַם זָבַ֨ת חָלָ֥ב וּדְבַ֛שׁ הִ֖וא וְזֶה־פִּרְיָֽהּ׃ (כח) אֶ֚פֶס כִּֽי־עַ֣ז הָעָ֔ם הַיֹּשֵׁ֖ב בָּאָ֑רֶץ וְהֶֽעָרִ֗ים בְּצֻר֤וֹת גְּדֹלֹת֙ מְאֹ֔ד וְגַם־יְלִדֵ֥י הָֽעֲנָ֖ק רָאִ֥ינוּ שָֽׁם׃ (כט) עֲמָלֵ֥ק יוֹשֵׁ֖ב בְּאֶ֣רֶץ הַנֶּ֑גֶב וְ֠הַֽחִתִּי וְהַיְבוּסִ֤י וְהָֽאֱמֹרִי֙ יוֹשֵׁ֣ב בָּהָ֔ר וְהַֽכְּנַעֲנִי֙ יֹשֵׁ֣ב עַל־הַיָּ֔ם וְעַ֖ל יַ֥ד הַיַּרְדֵּֽן׃
(25) At the end of forty days they returned from scouting the land. (26) They went straight to Moses and Aaron and the whole Israelite community at Kadesh in the wilderness of Paran, and they made their report to them and to the whole community, as they showed them the fruit of the land. (27) This is what they told him: “We came to the land you sent us to; it does indeed flow with milk and honey, and this is its fruit. (28) However, the people who inhabit the country are powerful, and the cities are fortified and very large; moreover, we saw the Anakites there. (29) Amalekites dwell in the Negeb region; Hittites, Jebusites, and Amorites inhabit the hill country; and Canaanites dwell by the Sea and along the Jordan.”
If the spies were telling the truth, why was it so bad?
Akeidat Yitzchak by Isaac Arama (quoted from Nechama Leibowitz's Studies in Bamidbar.
The crime of the spies is that instead of acting as neutral observers contenting themselves with the facts, they gave their opinion on the matter... The spies qualified the description of the conditions they saw by the significant word: "but", the people are fierce. In their report there was already a nuance of their own subjective feelings, regarding the possibilities of conquering the land....
If they started by praising it, why did they then change their tune?
(במדבר יג, כז) ויספרו לו ויאמרו באנו וגו' וכתיב אפס כי עז העם אמר רבי יוחנן (סימן אמ"ת לבד"ו לוי"ה) משום ר"מ כל לשון הרע שאין בו דבר אמת בתחילתו אין מתקיים בסופו

The Torah states: “And they told him, and said: We came to the land to which you sent us, and it also flows with milk and honey” (Numbers 13:27), and then it is written: “However the people that dwell in the land are fierce” (Numbers 13:28). Why did the spies praise the land and then slander it? ... Any slander that does not begin with a truthful statement ultimately does not stand, i.e., it is not accepted by others.

Why was it so bad to say "but"?
Rabbi A.L. Scheinbaum, Peninim on the Torah: Tenth Series
This is new form of lashon hara (gossip/slander). One speaks well of another person, even glowingly presenting his positive attributes; then, suddenly, he adds one word -- "but," and immediately everything changes. No longer are his virtues remembered. The "but" just accomplished that. With one word, all of the good information that he had previously related no longer carries any merit.