“The closing act of the great rebellion”
One hundred fifty years ago today, June 5, 1865, Federal forces formally took possession of Texas. Captain Benjamin F. Sands, commanding the division of the West Gulf Blockading Squadron stationed off Galveston, boarded a small Union steamer, U.S.S. Cornubia, and entered Galveston harbor, followed by another gunboat, U.S.S. Preston. Sands disembarked with a handful of other officers — but took no armed escort — and was met on the wharf by a Confederate officer. The officer escorted the Union men a few blocks to City Hall (above), where both Sands and the mayor of Galveston addressed a crowd that had gathered there. Both men made assurances of their goodwill and urged the population to go about their business peaceably. Sands told the crowd that he carried a sidearm that day not out of any fear for his own safety but as a sign of respect for the mayor and local officials. Then, along with the mayor, Sands continued on to the old U.S. Customs House, where he “hoisted our flag, which now, at last, was flying over every foot of our territory, this being the closing act of the great rebellion.”
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Dead Confederates, A Civil War Era Blog 
I read quite recently that in the summer of 1866 President Andrew Johnson declared that the insurrection in Texas was suppressed. Given your knowledge of Texas history, do you know what was going on in Texas that would cause Johnson to make such a statement more than a year after Appomattox? Was he simply referring to something along the lines of a handful of Confederate guerillas that were holed up in the mountains and robbing stagecoaches, or was there more to it than that?
I’m not familiar with Johnson’s declaration, but there was a lot of violence here in 1865-66, that was carefully documented by the Freedmen’s Bureau. It may have subsided somewhat, but flared again in 1868 in the run-up to the presidential election of that year.
This is covered, among other things, in Gregory Downs’ After Appomattox. I would say that it has more to do with Johnson’s struggles with Congress than with anything going on in Texas.