The Chinese had been fighting the Japanese before that, about 1933 I think.
well it's kinda complicated, and the plausible starting points look something like this:
3 May 1928 - Jinan incident, first actual shooting between GMD and IJA regulars, ended after 8 days by GMD accepting a cease-fire under Japanese pressure
18 September 1931 - Mukden incident, terrorist act engineered by IJA officers to provide a flimsy pretext for Japanese invasion of Manchuria and the set-up of a puppet regime on Chinese territory, sparking five months of fighting; first actual occupation of China by Japan (depending on how you feel about the Port Arthur and Qingdao concessions)
28 January 1932 - Shanghai War, another Japanese invasion sparked by a flimsily-manufactured IJA pretext, resulted in a month and a half of extremely heavy fighting ended unilaterally by the GMD declaring a cease-fire; Japan did not accept the cease-fire for several days until League of Nations pressure compelled it to accept a peace agreement
3 January 1933 - Battle of Shanhaiguan, prelude to a Japanese offensive on Rehe province south of the Great Wall, engineered by the IJA to secure a military buffer for Manchukuo; by late May, all allied Chinese forces had been compelled to retreat from the Great Wall
11 May 1933 - Invasion of Inner Mongolia, IJA effort following up on a filibuster by a collaborationist general a few weeks earlier, resulting in heavy fighting against nationalist Chinese forces organized independently of Jiang's GMD (which agreed to a formal truce with the Japanese at Tanggu on 31 May) for several months
14 November 1936 - Suiyuan incident, invasion of a province of Inner Mongolia outside Japanese control by filibuster troops organized by IJA advisors; comprehensively defeated by local forces and disavowed by both Tokyo and the Kwantung Army
7 July 1937 - Marco Polo Bridge incident, outbreak of badly confused fighting between IJA and GMD forces outside of Beijing; resulting in a full IJA offensive to conquer Beijing; by the end of the month, GMD forces had been forced to retreat behind the Yongding River, where the IJA stopped as the Konoe Fumimaro cabinet attempted to open negotiations with Jiang
9 August 1937 - Death of Japanese marine lieutenant Isao Ōyama, apparently shot by Chinese paramilitary police near an off-limits Chinese military airport; Ōyama's death broke up negotiations and led to an immediate military buildup by both sides in Shanghai (which had been demilitarized at the end of the 1932 war); on the afternoon of 13 August, the IJA formally crossed into Chinese territory, beginning major military operations that continued without a halt until 1945
9 December 1941 - Japan issues a declaration of war against the Republic of China
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Most authorities use the 9 July 1937 date for the outbreak of fighting, although 13 August 1937 is not unpopular.
Essentially nobody uses the date of the actual declaration of war, in 1941, because by that time Japan and China had had million-man armies slugging it out for four years, with millions more civilians dead. But that's what makes it hard to define when the war actually started, because the process of escalation only proceeded in fits and starts.
18 September 1931 is especially popular among Chinese nationalists, as is 19 September; 18 September for the actual terrorist act at Mukden, and 19 September for the Kwantung Army's invasion of Chinese Manchuria. To my mind, however, given the stop-and-start nature of fighting after the Mukden incident, the Jinan incident makes an equally good starting point. You could argue that the occupation of Manchuria was a significant break with the past, and it was, but Japan had been occupying significant chunks of China for decades at that point (Shandong especially, but also the Kwantung concession and a network of smaller districts throughout coastal China); Manchuria, obviously, was a lot bigger, but the point is that it's fuzzy.
This even applies to the 9 July issue; the Marco Polo Bridge incident did result in major ground-combat operations, but so did the first invasion of Shanghai back in 1932, and the fighting on the Wall in 1933, and even though the GMD wasn't involved there was major fighting between the Japanese and regular Chinese warlord forces for several months in 1933 in Inner Mongolia, and again for several months in 1936-37, and lower-level partisan warfare during the intervening years. There was a unilateral cease-fire after the Japanese occupation of Beijing and Tianjin starting on 31 July 1937, which held for two weeks before the Battle of Shanghai ended cease-fires for good.
Another dimension is that people attempt to draw lines between "China's war with Japan" and "Jiang's war with Japan" by arguing that they were not the same thing. Thus you will sometimes see people claiming that Jiang and his loyal forces only got into the fight in 1937, but other Chinese had been fighting Japan since 1931. This is, obviously, not true; Jiang's China was at war with Japan for a week in 1928 and for much of 1931-33, despite his preference to wait until later for war. It's true that from late 1936-37 Jiang was compelled to restructure his priorities and push for a united Chinese war against Japan, and few crises passed between that decision and the outbreak of major ground fighting. But then we get into other difficult territory: do we really say that the country of China and the country of Japan,
regardless of who was actually fighting and dying at the time, were only at war from the time one admittedly important Chinese guy decided to be at war? On the other hand, if you get too slippery with the dates you end up saying that the war started with the Treaty of Portsmouth or the Twenty-One Demands and well that just doesn't help anyone.
Periodization can suck sometimes.
