A few of those departments do indeed exist to defend this nation’s borders. Now, please keep in mind that I didn’t do an in-depth reading and study of the original documents before I made my comments ... but seems to me that defense of our borders and this nation is a founding principle.
U.S. Constitution - Article 1 Section 8
To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;
To provide and maintain a Navy;
To make Rules for the Government and Regulation of the land and naval Forces;
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
To provide for organizing, arming, and disciplining the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;