What does "fully mobilized" mean? Those sound kinda like weasel words you could make all kinds of arguments about a country not being "fully mobilized" so it doesn't count.

The Japanese beat the hell out of the Russians both on land (Mukden) and at sea (Tsushima) in the Russo-Japanese war in 1904-1905.

Before approaching the question, it must be made clear that a presumption exists - that the Japanese ‘developed delusions of grandeur’. I will presume by ‘delusions of grandeur’, you mean Japanese motivations for starting the wider Pacific War by attacking Pearl Harbour. It will be explained below that the Japanese high command never did develop delusions of grandeur, but was more than aware that a war against a ‘fully mobilised’ western enemy (the United States) would lead to defeat. Rather, it was this awareness of impending defeat that, paradoxically, led Japan to war.

But as the question is primarily enquiring whether the Japanese ever fought a ‘fully mobilised’ European military, I will respond to that first. As mentioned by u/jimros, we need to define what a ‘fully mobilised’ military means. I take it that by ‘fully mobilised’, you mean a ‘total war’, a war in which a nation devotes every resource, both military and civilian, to the war effort. In that sense, it is true that Japan technically never fought a fully mobilised European military after the Meiji Restoration. But in the Russo-Japanese War, the only full scale conflict between Japan and a European power pre-WWII, Russia did commit significant resources to the war that still ended in a Russian defeat.

In the Russo-Japanese War, Russia never fully mobilised her entire army. When the war started, Russia had only 150,000 men defending the east. General Kuropatkin, the overall commander of Russian forces in the East, calculated that the Russian army would need 5 to 7 months to reach its full strength on the East Asian battlefield, since the Trans-Siberian Railway could only provide one track for transports. This created a peculiar situation in which, despite major land defeats at the Yalu River, Liaoyang and Mukden that resulted in more than 200,000 casualties, the Russian army was at its peak of 700,000 men in the east at the end of hostilities in August 1905, with more troops arriving. Russia’s defeat came at the time when the military balance on land had tipped to her favour. The Russian army was never fully committed to the war. However, the partial mobilisation of Russia did not mean that the battles were small-scale. The climax of the war at Mukden saw the commitment of more than 250,000 Japanese and more than 300,000 Russians to the fight. Comparing it with the largest battle in the Franco-Prussian War, the Battle of Sedan, just the amount of Russian troops at Mukden outnumbered the combined strength of both the French and Prussian main armies at Sedan.

Unlike the army, a significant portion of the Russian navy participated in the war in the east. Most of the Russian Pacific Fleet based at Port Arthur and Vladivostok, with seven battleships, were sunk or blockaded at Port Arthur in the initial phases of the war. This resulted in the commitment of a large part of the Russian Baltic Fleet, including eleven out of thirteen battleships, to the east to contest the seas. This expeditionary fleet was promptly destroyed by Japan in the Battle of Tsushima. The complete defeat of the majority of Russian naval forces meant that in spite of a growing superiority on land, Russia could not hope to cross over to the Japanese homeland. Russia was forced to concede defeat and sue for peace. Therefore, we can see that Russia did commit significant land and naval forces to the war.

On an economic level, Russia had to strain her economy to continue the war. If we calculate the economic impact of war by the consumption of ammunition, in the seven month-long Franco-Prussian War in 1870-71, Prussia consumed 25 million rifle rounds. In comparison, in eleven days of fighting at the Battle of Mukden, 20.11 million rifle and machine gun and 279,394 shells were consumed. The expenses of modern war, including ammunition, food and supplies that all had to be sent from one end of the country to the other end by the Trans Siberian Railway, came as a surprise to the Russian government. Russia’s taxation system could not keep up with war expenses and the government was forced into applying for foreign loans. The economic scale of the Russo-Japanese War was uncomparable at that time to any other war previously fought. Although Russia never mobilised the home front, meaning the country never did enter a ‘total war’ phase, Russia did spend heavily on the war, and the poor economic situation by the end of the conflict contributed to the start of the 1905 Russian Revolution.

Finally, we must recognise that Russia is a unique European entity. Her economic power base and military strength was much larger than any other European country. Losing 300,000 men and almost their entire navy would be a disaster for any major European power, to say nothing about smaller nations such as Belgium, Serbia or Italy. It is true that Russia never did fully mobilise - but it is also true that Japan went toe to toe with a European power that committed significant forces and came out victorious.

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Sources:

Jacob, Frank, The Russo-Japanese War and Its Shaping of the Twentieth Century (London: Routledge, 2017).

Rethinking the Russo-Japanese War, 1904-5: Centennial Perspectives, ed. by Rotem Kowner (Leiden: Brill, 2007).

The Russo-Japanese War in Global Perspective : World War Zero, ed. by John W. Steinberg (Leiden: Brill, 2005).

If by "fully-mobilized," you mean a country that has called up all its reservists and fully shifted its economy to a wartime footing - then no, Japan hadn't done that by 1941.

Japan had plenty of experience fighting wars on a massive scale prior to 1941. Some of these experiences taught Japan some good lessons and some reasonable, but ultimately misleading ones.

Japan's stunning victory against enormous Russian forces in 1904-1905. It can be easy to forget the sheer scale of this war - here are just a few battles that illustrates the point.

Battle of Liaoyang (26 August to 3 September 1904) - Three Japanese Armies (1st, 2nd and 4th) with 128,000 men and 170 artillery attack five Russian corps (I, III, IV, V Siberian, and XVIII) with 158,000 men and 609 artillery pieces. The result is a Russian tactical victory, but Japanese strategic win, since the Russians are forced to retreat. The Japanese lose 5,540 killed and 18,600 wounded to 3,600 dead and 14,300 wounded Russians.

Even bigger land battles follow.

Battle of Mukden (19 February to 10 March 1905) - Five Japanese armies with 200,000 infantrymen, 7,350 cavalrymen, 925 artillery pieces, and 174 Hotchkiss machine guns launches a massive spoiling attack against three Russian armies about to go on the offensive. The Russians have 275,000 infantry, 16,000 cavalry, 1,439 field guns, and 56 Maxim machine guns. Nearly half a million soldiers took part in the battle - at the time, it was one of the largest battles in history. Nearly 30 percent of the men on both sides are killed or wounded in the decisive Japanese victory.

Battle of Tsuhima (27-28 May) - The Russians throw most of their Baltic fleet at the Japanese and lose badly - 8 Russian battleships are sunk or interred, along with more than a dozen destroyers and cruisers. This was on top of previous heavy Russian losses at Yellow Sea and Port Arthur. It is the largest naval battle of this period. In this battle and others, most of the Russian Navy is mobilized and committed to the war, and much of it is destroyed.

Neither side fully mobilized or shifted to a total war economy like that of WWII. But both sides put a massive amount of resources into the conflict. There were food riots in Japan over wartime shortages and Russia spent so much it got into economic trouble.

I think it's a bit far to say Japan's victory made Japanese leaders cocky - the war had been enormously costly in blood and treasure. However, it did teach Japanese generals and admirals some reasonable lessons.

The IJA became even more convinced a spirited infantry attack could succeed, even against great odds. Now, this isn't to say that human wave charges that trade bodies for ground become IJA doctrine! Instead the idea is that light artillery and machine guns can support a determined assault by infantry. Inculcating a "fighting spirit" in soldiers becomes even more important. The combat experiences of the Second Sino-Japanese War and other clashes in China only reinforced this lesson.

The Russo-Japanese War confirmed the IJN's faith in intense training and the value of having the modern technology, especially when it came to torpedoes. These values are a big part of why the IJN is so successful in the opening moves of 1941 and 1942.

Japan also fights the Soviet Union in the roughly division- and corps-sized engagements at the Battle of Khalkhin Gol in 1939 ... and loses. Japan quite reasonably concludes that the IJA really isn't ready for mechanized warfare against the Soviets. The Hokushin-ron ("Northern Road") expansion plan into Manchuria and the USSR is shelved in favor of the rival Nanshin-ron ("Southern Road") campaign into the Pacific and SE Asia.

If you want to talk about cockiness, then the early Japanese victories of 1941-1942 do lead to a case of senshoubyou ("Victory disease") and over-confidence. However, it's understandable why the Japanese felt this way - the fall of mighty Singapore, the capture of the entire Philippines, the stunning Pearl Harbor attack, and the sinking of Prince of Wales and Repulse all gave Japan very understandable reasons for optimism. On a strategic level, the Allies might not have been fully-mobilized by early 1942 - but Japan was taking on some of the best the Allies had to offer and winning. It's understandable why the Japanese got a case of the victory flu.