Behar תרל”ט

מצות שמיטה ושבתה הארץ שבת לה’.
The (reason for the) mitzvah of Shemitah is “The land will rest a Shabbos for Hashem” (Vayikra 25:2)
ובפ’ בחקותי אז תרצה הארץ כו’ שבתותי’.
In Bechukotai (Vayikra 26:34) (it states that the reason for the exile from Eretz Yisrael is because Bnei Yisrael did not keep the Shemitah) “And then the land will rest and make up for its Shemitah years”
כי השמיטה להראות כי הכל מאתו ית’ וכשבנ”י מבררין זאת ברצונם חל ע”ז ש”ש. וח”ו כשביטלו שמיטה הוכרח הבורא ית’ להראות בכחו כי הכל שלו ונגרשנו מארצנו.
This is because the reason for shemitah is to demonstrate that everything is from HaShem. When Bnei Yisrael clarify this on their own free will then the glory of HaShem rests upon their actions. However if Heaven forbid, they forsake the mitzvah of Shemitah then HaShem is forced to show with His power, how everything is indeed His, and we were exiled from our land.
אבל לא כ’ שבת לה’ רק בקיום השמיטה ברצון בנ”י כנ”ל:
However (in Bechukotai, when we were forced out of the land, and the land makes up for its shemitah years), it does not say that it is “a Shabbos to HaShem”, it is only when Shemitah is kept properly through the choice of Bnei Yisrael (that it is indeed a “Shabbos to HaShem”), this is as above. (that the real demonstration of HaShem’s glory comes about through Bnei Yisrael accepting the mitzvot of HaShem.)