Because Grant had laid siege to the town for several weeks, and the troops and civilians were starving.
As for how Grant had managed to gain the ascendancy, this was partly through his own tactical skill, and partly because the garrison commander (John Pemberton) was at the mercy of conflicting orders from his President and his area commander.
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Union General Grant wanted to capture Vicksburg Mississippi because it was a Confederate arsenal with a strategic position on the Mississippi River. It was a railway hub that distributed food from west of the Mississippi to locations in the South. Grant and earlier the Union navy tried to take Vicksburg without success. On the third campaign to capture the city it finally succeeded.
It was a common military strategy. The would starve them out, as going in at them full force and head on could result in many casualties and a big loss for the Union battle. It is a simple, cruel, yet work in answer to the situation. I know that it must sound cruel and mean and all, but that, a siege, has alwaysbeen around. This is and was certainly most meaningful tactic and it definitely promised answers and solutions to the problem at hand, as Grant pulled off yet another siege on Petersburg, successfully.
The largest surrender was Lee surrendering the Army of Northern Virginia to U.S. Grant at Appomattox Court House (April 9th 1865). It was not the last surrender. Joseph E. Johnston surrendered his army to Sherman later the same month, and the final surrender was at Palmito Ranch, Texas in May. PS. Checking the troop-numbers, I find that Lee surrendered only 28,000 men. In July 1863, Grant had captured 30,000 men after the siege of Vicksburg. So it could be that Vicksburg represented the biggest surrender.
What are the causes of when Lee met Grant and agreed to surrender ?
Port Hudson
Vicksburg was to valuable as a river port to destroy.
Ulysses S. Grant - apexxx.(: