The Annexation of Texas (1845) was the U.S. incorporation of the independent Republic of Texas as the 28th state, a victory for Manifest Destiny that triggered a border dispute with Mexico, helped cause the Mexican-American War, and reignited the sectional fight over slavery's expansion.
The Annexation of Texas is the moment the United States absorbed the Republic of Texas, an independent country since 1836, as the 28th state in 1845. Texas had won independence from Mexico nearly a decade earlier (after Santa Anna's surrender at San Jacinto), and most Texans wanted to join the U.S. right away. So why the nine-year wait? Slavery. Texas allowed slavery, and adding a huge new slave state threatened the sectional balance in Congress, so annexation sat on the shelf until expansionist momentum, and President-elect James K. Polk's 1844 victory on an annexation platform, finally pushed it through.
For the CED, annexation is the hinge between Manifest Destiny as an idea and expansion as a crisis. Advocates argued that Manifest Destiny and the superiority of American institutions compelled the U.S. to expand to the Pacific (KC-5.1.I.B), but annexing Texas provoked exactly the 'competition and violent conflict' that knowledge point warns about. Mexico had never recognized Texan independence, disputed the border (Nueces River vs. Rio Grande), and treated annexation as an act of aggression. That dispute gave Polk his opening for the Mexican-American War, which added even more territory and raised urgent questions about the status of slavery, American Indians, and Mexicans in the new lands (KC-5.1.I.C).
This term lives in Unit 5 (1844-1877) and threads through Topics 5.1, 5.2, and 5.3. It directly supports APUSH 5.2.A (causes and effects of westward expansion) because annexation is the clearest example of Manifest Destiny producing real territorial growth and real conflict. It supports APUSH 5.3.A (causes and effects of the Mexican-American War) because annexation is the proximate cause of that war. And it supports APUSH 5.1.A (the context for sectional conflict) because every acre gained from Texas and the war that followed forced Congress to answer the question it had been dodging since the Missouri Compromise. Will slavery expand into new territory? In short, annexation is where the expansion story and the slavery story collide, which is the whole engine of Unit 5.
Keep studying APUSH Unit 5
Manifest Destiny (Unit 5)
Annexation is Manifest Destiny in action. The ideology said America was destined to stretch to the Pacific; Texas in 1845 was the first big proof that voters and politicians would actually act on it. If an MCQ asks what ideology the annexation reflects, this is the answer.
Treaty of Guadalupe Hidalgo (Unit 5)
Annexation starts the chain that the treaty finishes. Annexation angered Mexico, the border dispute sparked the Mexican-American War, and the 1848 treaty handed the U.S. the Mexican Cession. Think of annexation as the first domino and Guadalupe Hidalgo as the last.
Abolitionist Movement (Units 4-5)
Abolitionists and many northern Whigs opposed annexation because Texas would enter as a giant slave state. Their resistance is why annexation stalled from 1836 to 1845, and it previews the free-soil politics that dominate the rest of Unit 5.
Alaska & Hawaii Expansion (Units 6-7)
Texas sets the template for later U.S. expansion. Americans settle or invest in a territory, the U.S. annexes it, and questions follow about the people already living there. Exam questions love asking you to spot this recurring pattern in American expansionism.
Annexation of Texas shows up most often as a cause-and-effect MCQ. Stems ask what ideology it reflects (Manifest Destiny), what it directly prompted (the border dispute and Polk's war message to Congress), or what broader pattern of American expansionism it illustrates. You need to be able to run the causal chain in both directions: Manifest Destiny → annexation → Mexican-American War → Mexican Cession → sectional crisis over slavery in the territories. No released FRQ has used the term verbatim, but it's prime evidence for causation essays on the coming of the Civil War and for continuity-and-change arguments about U.S. expansion. A DBQ on sectionalism or expansion almost always rewards Texas as outside evidence, as long as you connect it to the slavery question rather than just naming the date.
These are two different events nine years apart. In 1836, Texas won independence from Mexico and became its own country, the Republic of Texas. In 1845, the U.S. annexed that republic as the 28th state. Mexico never accepted the first event, which is why the second one led to war. If a question mentions Santa Anna or the Alamo, you're in 1836; if it mentions Polk or statehood, you're in 1845.
The U.S. annexed the independent Republic of Texas as the 28th state in 1845, nine years after Texas won independence from Mexico.
Annexation was delayed for nearly a decade because adding a large slave state threatened the sectional balance in Congress.
Annexation is the textbook example of Manifest Destiny producing both territorial growth and violent conflict (KC-5.1.I.B).
Mexico never recognized Texan independence, so annexation plus the Nueces-Rio Grande border dispute led directly to the Mexican-American War.
The territory gained from annexation and the war raised the question of slavery's expansion, which drives the sectional crisis across the rest of Unit 5.
On the exam, always frame annexation as a cause: of the Mexican-American War, of the Mexican Cession, and of the slavery debates that followed.
It was the 1845 incorporation of the Republic of Texas into the United States as the 28th state. In APUSH it matters as a product of Manifest Destiny and a direct cause of the Mexican-American War and the sectional fight over slavery's expansion.
Essentially, yes. Mexico had never recognized Texan independence and considered annexation an act of aggression, and the dispute over whether the border sat at the Nueces River or the Rio Grande gave Polk the clash he used in his war message to Congress in 1846.
Texas independence (1836) is when Texas broke away from Mexico and became its own country after Santa Anna's surrender. Annexation (1845) is when the U.S. made that country a state. Don't merge them into one event; the nine-year gap exists because of the slavery debate.
Slavery. Texas would enter as a huge slave state, and northern politicians and abolitionists feared it would tip the balance of power in Congress toward the South. Polk's expansionist win in the 1844 election finally broke the logjam.
Yes. It anchors Topics 5.1-5.3 and learning objectives APUSH 5.2.A and 5.3.A, and it shows up in multiple-choice stems about Manifest Destiny, the causes of the Mexican-American War, and patterns in American expansionism.