Around the first week in January, 1861,the arsenal at Augusta was taken by Georgia troops without firing a shot.
The U.S. Arsenal at Apalachicola was occupied by Florida troops, the caretakers having walked out. The U.S. arsenal at Baton Rouge was taken by Louisiana troops along with Forts Phillip, Jackson and Pike, and Macomb. Fort McRae and the naval yard at Pensacola were occupied by Florida troops. Fort Massachusetts was occupied by Mississippi troops.
All were done peacefully while arranging for the Federal caretakers to ship home.
Charleston Harbor's main fort, Fort Moultrie, along with two of the other three major forts there all predated the federal presence. Similar forts were found in all of the original British colonies in the south (VA, NC, SC, GA) and also several of the gulf states. There were defensive positions originally built by the Spanish all over Florida. The harbor defenses of Mobile, Alabama were begun back in 1699 and built over the following century. Even the “new” forts around Mobile by the civil war had pre-federal components as all US army improvements were built upon existing structures.
Louisiana was another classic example as practically every major fort in the state was a pre-federal structure. The main defense of New Orleans and the Mississippi river mouth, Fort St. Philip, was begun in 1792 by the Spanish on an earlier French fort begun in 1761. The other river mouth defense, Fort Jackson, was originally Fort Bourbon from pre-American days and had only been modernized and built upon by the feds. Fort St. Philip was used prominently in the War of 1812 and in the civil war battle of New Orleans. Several of the other city defenses were built by the Spanish or French and one of its forts on the Mississippi was even built by pirates.
Texas was another case of a state dominated by pre-federal forts. Most of the state's forts dated from the Republic of Texas days or earlier. A few far western posts there were built by the federal government in the late 1840’s, but these tended to be little more than wooden stockades. Most of the central Texas stockades were pre-federal frontier defenses and virtually all of the stone forts in the state dated back to Republic of Texas or Spanish control.
Your accusation of aggressiveness is unfounded.
It's more than simply well founded - it's considered conventional wisdom.
But if a robber walks into a bank, speaks politely to all while pointing a gun & demanding the loot, and then escapes without hurting anyone -- it is still a bank robbery, still unlawful, still subject to criminal prosecution and penalties.
The fact is that secessionists seized dozens of major Federal properties, including forts, ships, arsenals and mints.
If you wish to argue that some of those were not actually Federal property, that they had always been state property, then in fact they were not "seized", merely re-occupied.
But that is not what history records.
History says those Federal properties were "seized" by force, some even before their states formally declared secession.
It makes those occupations illegal and provocations for war.