Jeremiah 10:10

וַה’ אֱלֹקים אֱמֶת הוּא אֱלֹקים חַיִּים וּמֶלֶךְ עוֹלָם מִקִּצְפּוֹ תִּרְעַשׁ הָאָרֶץ וְלֹא יָכִלוּ גוֹיִם זַעְמוֹ

Hashem G-d is true; He is the living G-d and the eternal King. The earth trembles at His wrath and the nations cannot stand against His fury.

At the end of Shema, we append the words “Hashem Elokeichem” (Hashem your G-d) to the word “emes” (true) that begins the next paragraph. This is done to paraphrase this verse from Jeremiah. Rashi explains that Hashem is the living G-d, the King of the world, therefore when He says something, He can make sure it comes true. This is not the case with humans, whose plans are often thwarted by circumstances beyond their control.