There have been many historical “lines in the sand.” The Spanish explorer Pizarro is said to have a “line in the sand” to his troops in Peru.
 
Lieutenant Colonel William Travis is said to have drawn a line on the floor with his sword at the Alamo in March 1836. All who wanted to stay at the Alamo and fight would have to cross the line. Colonel Bowie was too sick to cross, but was pulled across in a cot. All crossed the line except one person, who escaped (and told the story).
 
President George Bush (in 1990) said that he drew “a line in the sand” against Iraq invading Saudi Arabia. The Alamo reference must have been known to Prsident Bush when the statement was made, just before the start of the first Gulf War.
 
 
Alamo City Guide
The Battle Of The Alamo
Unsheathing his sword during a lull in the virtually incessant bombardment Colonel William Barret Travis drew a line on the ground before his battle-weary men. In a voice trembling with emotion he described the hopelessness of their plight and said, “those prepared to give their lives in freedom’s cause, come over to me.”

Without hesitation, every man, save one, crossed the line, Colonel James Bowie, stricken with pneumonia, asked that his cot be carried over.
 
Google Books
A Line in the Sand: The Alamo in Blood and Money
by Randy W. Roberts and James Stuart Olson
New York: Simon and Schuster
2001
Pg. 155:
She remembered that on March 5, 1836, Travis addressed his troops. She forgot exactly what he said but recalled vividly that “he drew a line on the floor with the point of his sword and asked all who were willing to die for Texas to come over on his side.” All but two stepped over the line. One of the two “sprang over the wall and disappeared.” The other was Bowie, and he was just too weak to step at all. “He made an effort to rise, but failed, and with tears streaming from his eyes he said: “Boys, won’t none of you help me over there?” Crockett and a few others came to his aid. “At the time,” she commented, “we all knew that we were doomed, but not one was in favor of surrendering.”
 
Madam Candelaria was not the only person—or even the first—to mention Travis’s last speech to his men. WIlliam P. Zuber, who at the time of his death, in 1913, was the last surviving member of General Sam Houston’s army, told a similar story.
(...)
Pg. 156:
Did Zuber invent the Rose story? Perhaps. Even he later confessed that he had invented some of Travis’s speech, though he insisted that the story was true and his rendition of the address was faithful to the actual one. In truth, there probably has been a Louis Rose at the Alamo, and he undoubtedly did depart the mission before the final battle. But historians have traditionally regarded the Rose-Zuber account of the line episode with hearty skepticism, charging that it is too melodramatic and too improbably to be true. The idea that Rose told Zuber’s parents, they told him, and he waited until 1873 to tell the world seemed far fetched at best, a damn lie at worst. And the critics might just be right.
 
But they also just might be wrong. The idea of clarifying a choice by scratching a line in the dirt and then asking men to cross to one side or another was hardly novel. Southerners often voted in this method, and Ben Milam supposedly rallied Texans to take Bexar in December 1835 with just such an action. Certainly it was a melodramatic gesture, but it was a melodramatic age and Travis was far more melodramatic than most.
 
Duel of Eagles:
The Mexican and U.S. Fight for the Alamo
by Jeff Long
New York: William Morrow and Company, Inc.
1990
Pg. 233:
In those days, militia officers (like Capt. Abraham Lincoln in 1832) were frequently elected by soldiers who physically voted with their bodies, crossing a line or standing in a group to show their support for a man or an issue.  Some of the men crossing Travis’s line had already rossed a similar line drawn by Ben Milam in December. The sight of a military unit mustering to one side or another of a line in the dirt would have struck no one as extraordinary. Rather it appealed tot heir democratic tastes. They viewed it for what it was, an election.
   
Wikipedia: Battle of the Alamo
Line in the sand
A legend exists that on March 3, March 4, or March 5, Lieutenant Colonel Travis drew a line in the sand with his sword and invited all those who were willing to stay, and presumably to die, to cross over the line. Allegedly, the invalid Jim Bowie was carried across the line at his request. According to one variant of the story, all but one defender crossed the line.

Louis Rose, said to be a French soldier who had fought under Napoleon in Russia before arriving in Texas, allegedly slipped out of the Alamo. After evading the Mexican forces by moving at night, Rose is said to have taken shelter with the family of William P. Zuber to whom he told the tale of his escape. In 1873, Zuber (his son) published a version of the story, which has not been historically documented. The phrase “drawing a line in the sand” has remained part of English jargon for taking a stand with no compromise.
 
Google Books
The History of America
by William Robertson
London: W. Strahan
1780
vol. 3; 3rd edition
Pg. 11:
Pizarro drew a line upon the sand with his sword, permitting such as wished to return home to pass over it, only thirteen of all the daring veterans in his service had resolution to remain with their commander (1).
(1) Herrera dec. 3 lib. x. c. 2, 3. Zarate, lib. i. c. 2. Xerez, 181. Gomara Hist. c.109. 
   
Google Books
History of the Conquest of Peru
by William Prescott
1847
PP. 87-88:
A ray of hope was enough for the courageous spirit of Pizarro. It does not appear that he himself had entertained, at any time, thoughts of returning. If he had, these words of encouragement entirely banished them from his bosom, and he prepared to stand the fortune of the cast on which he had so desperately ventured. He knew, however, that solicitations or remonstrances would avail little with the companions of his enterprise; and he probably did not care to win over the more timid spirits who, by perpetually looking back, would only be a clog on his future movements. He announced his own purpose, however, in a laconic but decided manner, characteristic of a man more accustomed to act than to talk, and well calculated to make an impression on his rough followers.
 
Drawing his sword, he traced a line with it on the sand from east to west. Then turning towards the south, “Friend and comrades!” he said, “on that side are toil, hunger, nakedness, the drenching storm, desertion, and death; on this side, ease and pleasure. There lies Peru with its riches; here, Panama, and its poverty. Choose, each man, what best becomes a brave Castilian. For my part, I go to the south.” So saying, he stepped across the line. He was followed by the brave pilot Ruiz; next by Pedro de Candia, a cavalier, born, as his name imports, in one of the isles of Greece. Eleven others successively crossed the line, thus intimating their willingness to abide the fortunes of their leader, for good or for evil. Fame, to quote the enthusiastic language of an ancient chronicler, has commemorated the names of this little band, “who thus, in the face or difficulties unexampled in history, with death rather than riches for their reward, preferred it all to abandoning their honor, and stood firm by their leader as an example of loyalty to future ages.”
 
History of the Conquest of Peru
Prescott must be quoting an earlier source. He refers to “an ancient chronicler” and Pizarro would have said something more akin to, “Alla esta Peru y aqui esta Panama. ...”
   
26 November 1853, Gleason’s Pictorial Drawing-Room Companion, pg. 350:
A couple of well-armed guides came upon us and insisted upon disturbing our meditations with their magpie chatter. (...) Talking did no good, they talked so much faster and in more languages than
we.  I remembered the brave American adventurer, and drew a line in the sand; if they crossed that, I hardly know what terrors were to come upon them.  And these stout, well armed men marched like lions to the line—there they stopped, and we were molested no more.
 
9 August 1990, New York Times, pg. A1:
BUSH DRAWS “LINE”
 
He Rules Out an Invasion
of Kuwait—Troops
Take Up Positions
By R. W. Apple Jr.
(...)
“A line has been drawn in the sand” against any Iraqi invasion of Saudi Arabia, Mr. Bush said.
 
26 August 1990, New York Times, “War Words” by William Safire, pg. SM18:
“A LINE HAS BEEN DRAWN in the sand,” said the President in a televised speech to the nation.
(...)
During the siege of the Alamo in San Antonio in 1836, legend has it that William Barret Travis drew a line in the ground (some say sand) with his sword and said, “Those prepared to die for freedom’s cause, come across to me” (189 of 190 Texas defenders did; the 190th provided the story).