What does the State of Texas have to do with an incident at a Chinese bridge in the 1930's?
Once there was no Texas. The area was inhabited, probably sparsely, by groups of Native American Indians. Then the area became part of the Spanish colonial possession Mexico. The Spanish settlement was also relatively sparse, but included notably included the town of San Antonio. The Spanish traded the Louisiana Territory, excluding Texas, to France, who immediately sold it to the United States. Disputes about boundaries between this territory and Spanish Texas were supposed to be settled in 1819. The United States wanted all of Florida as a slave state. Unable to defend Florida against the slaver-predator Andrew Jackson (later founder of the Democratic Party and President of the United States), Spain gave Florida to the U.S. and received, in part, undisputed title to Texas in return. In 1821 Mexico became independent of Spain, and included Texas. Few Mexicans wanted to move to Texas, but many Americans did. In 1835 and 1836 the Americans in Texas revolted against Mexico and became an independent nation, the Republic of Texas. With the consent of the government of Texas, the United States annexed them in 1845, and Texas on January 1, 1846 became a state of the United States. A state where slavery was legal.
A problem was that the border between Mexico and Texas was disputed. Another problem was that greedy men in the United States coveted Mexican lands, in particular California. The United States needed a pretext for war with Mexico so it could grab the disputed lands of south Texas, plus New Mexico, Arizona, California, etc. Mexican troops, a few of them anyway, were stationed between the Nueces and Rio Grande Rivers. President of the United States James Polk, claiming Texas extended to the Rio Grande, in 1846 purposefully sent a patrol of U.S. troops into the disputed territory, where they clashed with the Mexican army and lost the initial skirmish. The United States and Mexico then declared war on each other. (See the Thornton Affair at Wikipedia)
Polk claimed Mexicans "shed American blood upon American soil." Thus, in the American version, it was not a war of aggression, even if the U.S. "won" about 1/3 of Mexico through the war.
Now consider the Marco Polo Bridge incident, which occurred in China, near Peking (or Beijing), in 1936. Here began the war between Japan and the head of a coalition of Chinese war lords, Chiang Kai-shek, that eventually merged into the war between U.S. and Japan that ran in parallel to World War II. The American interpretation of the incident and war is that the Japanese were aggressors, that Chiang Kai-shek's government was the legitimate representative of the Chinese people (although no one was ever allowed to vote on that idea), and that the U.S. acts of war that proceeded the battle of Pearl Harbor were justified by Japanese aggression in China.
To put the incident in context, you need to understand a little about the history of Manchuria, an area in the extreme northeast of modern China that had become part of China when the Manchus conquered China back in the 1600's. Attacked throughout the 19th century by imperialist powers including the both the U.S. and Japan, by 1910 China was in a state of anarchy, with only nominal national governments, the reality being rule by local war lords. Russia, China, and Japan all wanted to control Manchuria, and of course at least some of the Manchurians would have preferred independence from all three powers. So Texas can be seen as a sort of American Manchuria, an area that was fought over by the U.S. and Mexico, but that also had a independent streak.
Two Japanese colonels, Kanji Ishihara and Seishiro Itagaki, with leftist and democratic views tried to turn Manchuria into a utopia starting in the 1920's. They used the Japanese army there, known as the Kwantung Army, to gain control of Manchuria without the permission of army headquarters or the Japanese government. The reality already was that under Japanese guidance the area was prospering economically and attracting many immigrants from China, Japan, and Korea. In 1931, with a name change to Manchukuo, the area became independent.
Unfortunately Ishihara and Itagaki lost control of the Kwantung Army, and were replaced by Kenji Doihara, who wanted to rule China, and who was backed by the expansionist, right-wing Control Clique of the Japanese army. He was able to create another autonomous, self-governing area south of Manchukuo. Chiang Kai-shek was mad that the war lords of this area were cooperating with the Japanese instead of with his southern-China based gang of war lords.
Which brings us to the Marco Polo (Lugou) Bridge, located 8 miles southwest of Beijing, and within the northern autonomous area. Japanese troops were in the area under the usual excuse of protecting the lives of Japanese citizens in the area. Accusations vary, but apparently Chiang thought he was strong enough to kick out the Japanese, so he had his local war lord Sung Chi-yuen, or perhaps some select group under Sung, attack the Japanese without provocation. It took a while for things to escalate. At first the Japanese government thought it was merely sending a punitive expedition of the type the U.S. sent to China during the Boxer rebellion. But the situation turned into an all out war between Japan and the two main government groups in China, the communists and nationalists.
I tend to side with the Mexicans and Chinese on this one. The Japanese had their points, including that Japan and Japanese occupied areas were better governed and more economically successful than the areas of China ruled by Chiang Kai-shek and other war lords. But they were acting as aggressors in a foreign nation. Invitations from local Manchu and other Chinese leaders existed, but that has been a fig leaf for aggression all to often in history. The situation was similar when an American puppet in South Vietnam invited the U.S. to attack North Vietnam decades later.
I see no mitigating circumstances at all regarding the start of the Mexican American War. The disputed area of Texas was, in fact, disputed, and grabbing it would have been one thing. But it was not really the area the U.S. government wanted to grab. It was a pretext for grabbing a vast area that had nothing to do with the disputed area.