Twiggs knew all along that he was going to side with the Confederacy. He warned General Scott on January 15, 1861 that he would go with Georgia when she left the Union and Georgia seceded 4 days later.
San Antonio, January 15, 1861. Lieut. Gen. W. Scott, U. 8. Army, Washington:
General : Yours, through Colonel Lay, of the 29th [28th] December is received. I am placed in a most embarrassing situation. I am a Southern man, and all these States will secede. What is left will not be the u United States,?? and I know not what is to become of the troops now in this department.
Louisiana, Alabama, Mississippi, and Georgia will certainly secede. As for coercion, that I consider impossible to keep them in the Union. A guard would have to be put at every house in the country, and that would not keep them in the Union. The feeling is universal, and the people are determined to secede. Coercion might have done at first; now it cannot. As soon as I know Georgia has separated from the Union I must, of course, follow her.
I most respectfully ask to be relieved in the command of this department on or before the 4th of March next. All I have is in the South, and as my health will not allow me to take an active part in the scenes that will probably be enacted, I must be a looker-on.
I am, General, with the greatest respect and regard, your obedient servant,
D. E. TWIGGS,
Brevet Major-General, U. 8. Army.
O.R. Series I, Vol. I, Ch. I, pg.581
On February 7, 1861 Twiggs met with Confederate commissioners and told them he would surrender all United States troops and property. When Ben McCulloch and his men show up in San Antonio on February 16, 1861 to demand Twiggs' surrender along with all Federal soldiers and Federal property, he feigns surprise in front of his troops although he already knew what was coming. Twiggs leaves for New Orleans and is given a Confederate hero's welcome upon arrival, being promoted to Major General, CSA on May 22, 1861.