not dependent on anyone

 וְכִ֣י תֹאמְר֔וּ מַה־נֹּאכַ֖ל בַּשָּׁנָ֣ה הַשְּׁבִיעִ֑ת הֵ֚ן לֹ֣א נִזְרָ֔ע וְלֹ֥א נֶאֱסֹ֖ף אֶת־תְּבוּאָתֵֽנוּ

 וְצִוִּ֤יתִי אֶת־בִּרְכָתִי֙ לָכֶ֔ם בַּשָּׁנָ֖ה הַשִּׁשִּׁ֑ית וְעָשָׂת֙ אֶת־הַתְּבוּאָ֔ה לִשְׁלֹ֖שׁ הַשָּׁנִֽים

Apparently the concern for having enough food during shemita is not something trivial, as Hashem responds with a promise that miraculously there will be a bumper crop the previous year.  But why should people worry?    Couldn't they just import produce from neighboring countries?  

Most if not all countries are rich in some resources but lack others.  They are dependent to some degree or other on trade with their allies and neighbors to make up the difference.  You scratch my back, I'll scratch yours.

Rav Charlap writes that Eretz Yisrael is different.  The only dependency Eretz Yisrael has is on Hashem.  Even at the cost of making miracles, Hashem does not allow for a situation where Eretz Yisrael would have to rely on its neighbors.

(26:5)  וְהִשִּׂ֨יג לָכֶ֥ם דַּ֙יִשׁ֙ אֶת־בָּצִ֔יר וּבָצִ֖יר יַשִּׂ֣יג אֶת־זָ֑רַע וַאֲכַלְתֶּ֤ם לַחְמְכֶם֙ לָשֹׂ֔בַע וִֽישַׁבְתֶּ֥ם לָבֶ֖טַח בְּאַרְצְכֶֽם Abarbanel, Netziv, and others interpret the end of the pasuk not as a bracha for protection against enemies attacking, but rather as a continuation of the beginning bracha for prosperity.  As a result of having all that you need in Eretz Yisrael proper, you won't have to travel elsewhere to seek out resources that are lacking.  As Ksav v'haKabbala puts it:

אינו יעוד השלום שלא ילחמו בהם אויביהם, רק הבטחה, שמפני רוב השובע שתהיה בארצם לא יצטרכו ללכת נע ונד בארץ אחרת לבקש מאכל כמו שכתוב נודד הוא ללחם איה

Hopefully we will be zocheh to see this bracha fulfilled in our lifetimes.