Was it still under Portuguese control? What efforts were made to maintain its neutrality? Did the Japanese attempt to occupy, blockade, or otherwise control it at any point during the war? Were Europeans or Americans in Macau interned or put into camps?
I gave a short answer to this question here a few years ago.
Essentially, Macau was controlled by Portugal, which was a neutral party to WWII. Japan honored that neutrality. That neutrality didn't stop Japan from having de facto control over Macau anyway, however... Portugal had no way of stopping Japan from stationing "advisors" there, for example.
Greetings! Alongside u/When_Ducks_Attack's excellent overview and linked response, this comment will go a tad more in-depth on why Japan (for the most part) respected Portugal's neutrality in the war. This sub-question is particularly interesting when you consider that the Japanese did occupy Portuguese Timor, going so far as to actively land troops there in 1942 (though the two nations did eventually come to an agreement of Japanese occupation being temporary and in-line with the defense of the nation). Let's begin.
Main Question
The first thing we have to note is the actual status of the mother country to which Macau belonged: Portugal. At the time of the Second World War, Portugal was under the fascist dictatorship of António de Oliveira Salazar, who simultaneously held the posts of premier, foreign minister, and war minister (a most impressive and practically all-powerful trio in regards to foreign policy). Salazar did not wish for Portugal to become an active combatant in the Second World War, though he did allow the country to become a "key interest" for both the Axis and Allies. For one, Portugal represented a key "listening post" for all of the nations involved, and a center of intelligence operations for all of them. In 1943 for example, the officially listed missions to Portugal (made up of various embassies and legations) from the belligerent states was far out of proportion to the actual size of the country:
The Portuguese authorities were in close contact with the Axis legations, though all of foreign contingents were monitored through clandestine means in one way or another. Salazar also had a key asset in the form of wolframite, otherwise known as wolfram ore, a major source of tungsten required in the steel-hardening process which forged all manner of weapons. The key sources of this rare material, other than neighbouring Spain, was the Portuguese colony of Mozambique. With all of these cards to play, the Japanese were acutely aware that angering Salazar by occupying Macau would lead to consequences which no diplomat was willing to endure.
Then again, by nature of Macau's situation in 1942, it was not in a position to exert neutrality that much either. The fall of Hong Kong and the progress of the Second-Sino Japanese war meant that it was a literal and metaphorical European island in a sea of Japanese interests. Historian Geofrrey C. Gunn on what this meant for the tiny colony:
"Virtually an island surrounded by Japanese land and naval forces, and served with a series of ultimatums, Macau was utterly beholden [reliant on] to Japanese permissions for all its external needs, including the movement of goods as well as people—which included Portuguese and other “neutral” or third-country nationals, especially those arriving from Hong Kong"^(1)
The Japanese were not willing to escalate matters further, though they did station soldiers s at the border gates of the colony, and Kempeitai (the fearsome Japanese equivalent of the Gestapo secret police) staff as well as IJA officers were common visitors to the city. At the official level, Portugal's colony governor Gabriel Teixeira often had to deal with the ultimatums and diplomatic moves of the Japanese consul Fukui Yasumitsu, whose assassination in February 1945 led to a debacle in relations between the two countries, but Tokyo received the clear signal that an occupation of Macau would lead to an unraveling in the Portugal-Japan relations.
Economically, Macau also served as a convenient middleman for Portugal to extract "hard currency" with which to pay for the movement of tungsten from interior parts of China for export to the home islands. Japan also had a stake in the aviation gasoline exported from Macau, as well as rice supplies which led to serious starvation and shortage in the colony from 1943 onwards.
So in conclusion, Macau's status during the Second World War can best be described as a "useful neutral", or to use the more historiographical term, "collaborating neutrality". Although the Japanese government respected Portuguese sovereignty over the island in formal protocols and agreements, fearing the loss of Lisbon as an intelligence center, Macau served as a conduit for economic efforts as well as a compliant bit of land where communication and transport were heavily regulated by the Japanese control of southeast China's waterways.
Part 1 of 2
Followup question answer:
During the first part of the Second Sino-Japanese War, 1937-1941, did Japan try to institute a blockade against China? If so, did the US/UK respect that blockade? What role, if any did Macau (and Hong Kong) have in the import of fuel and arms to the Chinese during this period? And thus I presume any such imports to Macau would have been shut down in December 1941, if not earlier.
The simple answer, in three parts, is:
In more drawn out and comprehensive terms:
1: Very shortly after the Marco Polo bridge incident which sparked the Second-Sino Japanese War Hasegawa Kiyoshi, Commander-in-Chief of the Imperial Japanese Navy's Third Fleet, announced a blockade of China's coast from Shanghai to Swatow (dated August 25th, 1937). On September 3rd, this blockade was later expanded to include the entire Chinese coastal region. In swift succession, the IJN began to secure its lines of communication by occupying and bombarding (in order) the Pratas Reef, Hainan Island, and Quemoy Island. By December of 1937, the entire north of the South China Sea (Taiwan to the entrance of the Gulf of Tonkin), was under the firm control of the IJN. Although France, Britain, and the US would later protest (though weekly and with some delay) the occupation of the Spratly Islands and the potential invasion of Hainan Island, the IJN retained practical control over the flow of goods into and out of China until the war expanded to include Britain and America.
2: The British and American governments did comply with the Japanese blockade for the most part, realising that the war in Asia between the two powers was a commitment that neither of them wished to get too deep into (fearing of course, that such movements would lead to direct war). The British were actually trying to get the United States to add their weight behind appeals for diplomatic negotiations between China and Japan (most notably when Shanghai fell on the 14th of August). The American government however, was unwilling to to do, as Historian Andrew Kelly notes:
"It was more apparent than ever before that the United States was not willing to become directly involved in the Sino-Japanese War. Indeed, the United States seemed to believe that unilateral action - or rather - unilateral inaction - was more effective in addressing the Far Eastern Crisis than was cooperation with the British."^(2)
The crisis of the Second-Sino Japanese War for both powers took place on the 12th of December 1937, when Japanese planes attacked and sunk the USS Panay and damaged HMS Ladybird along the Yangtze River. There was a brief debacle in diplomatic correspondence between both countries and Japan, the main question being over whether the attacks had been accidental or not (the Japanese pilots claimed that the American flags on the deck were not visible from their altitude, a questionable claim). The matter quickly settled itself with the Japanese formally apologising and agreeing to pay an indemnity to the American government.
3. Hong Kong was a key refuge not just for civilians fleeing the war in China, but also for Chinese industry and businesses. Before the Japanese invasion in December 1941, there were 1,250 factories with over 90,000 workers (many of them having fled from Chinese cities as the war neared). Exports rose likewise, but the Japanese occupation brought a swift end to all that.
Hope this answers your followup somewhat, and feel free to ask any follow-ups!
Quotations
[1]: KELLY, ANDREW. "THE SINO-JAPANESE WAR AND THE ANGLO-AMERICAN RESPONSE." Australasian Journal of American Studies 32, no. 2 (2013): 27-43. Accessed February 4, 2021. http://www.jstor.org/stable/43863854.
Sources
GRANADOS, ULISES. "JAPANESE EXPANSION INTO THE SOUTH CHINA SEA: COLONIZATION AND CONFLICT, 1902—1939." Journal of Asian History 42, no. 2 (2008): 117-42. Accessed February 4, 2021. http://www.jstor.org/stable/41933494.
Meng, Chih. "Some Economic Aspects of the Sino-Japanese Conflict." The Annals of the American Academy of Political and Social Science 199 (1938): 233-42. Accessed February 4, 2021. http://www.jstor.org/stable/1021044.
Shuyong, Liu. "Hong Kong: A Survey of Its Political and Economic Development over the Past 150 Years." The China Quarterly, no. 151 (1997): 583-92. Accessed February 4, 2021. http://www.jstor.org/stable/655255.
Bayly, Christopher and Harper, Tim. Forgotten Armies: Britain's Asian Empire & The War with Japan. Penguin Books, 2004.