Before the war, the Japanese needed room for population expansion as they saw it (almost identical to Hitler's 'Lebensraum'). Japan also had a dire need of materials since Japan has very little by way of natural resources. There were all these neighbouring Asian countries, rich in resources and weak in military strength so the inevitable happened, Japan started to colonise other Asian countries. Eventually Roosevelt saw what was happening and sought to curtail their activities by imposing trade sanctions (no oil, no steel etc. for the Japs) and freezing their financial assets. Japan was screwed, it had to find a way to sustain its imperialist program, capturing foreign assets, but the Yanks above all were likely to stand in their way. Now Japan could not simply declare war on other major countries because it was already stretched with its campaigns on mainland Asia but they needed a way to bring about an understanding with other world powers to allow them certain conquests so as to fulfil their dreams of Empire. Remember they thought they were the other 'master race'. The real villain in all this was Hideki Tojo a military man who eventually became Prime Minister and who was also eventually hanged as a war criminal (though why they never hanged that four-eyed get Emperor Hirohito I'll never know - actually I do but that's another story. It was Tojo above all other Japanese who was responsible for Japan's Hitler-like fantasy of World domination.
Back to Japan's aggression against the US. They thought if they could cripple the US Pacific fleet that the US would not be able to retaliate against further Japanese 'colonisation' in Asia for some time and that by the time the US got back to strength it would be too late to reverse Japan's conquests - at which point Japan could then cease hostilities and negotiate a settlement. But they needed to take the US by surprise, hence the attacks on Pearl Harbour and the other US bases on the same day in 1941. The Japanese government was so full of itself at this stage (and desperate for raw materials) that they would not listen to any advice they were given by other knowledgeable Japanese including Admiral Yamamoto who orchestrated the Pearl Harbour attack (he put loyalty to his superiors above his own commonsense) so the attacks proceeded. The Japanese thought they were simply buying time with their attack against the US, make hay while the sun shines, then back off once the US had eventually recovered.
The backbone of Japanese strength was its Navy, they had superior naval strength to the US at the time and sought to smash US naval assets in the Pacific once and for all. They actually did a fairly good job at Pearl Harbour because their primary target was battleships and they got them. But as it turned out it was aircraft carriers that were the capital decisive ships and I have never been able to work out why the Japanese didn't realise that. There were no US aircraft carriers in port at the time of the Pearl Harbour attack. Even more curious because the Japanese were pioneer users of aircraft carrier technology which they originally got from the British after WWI. But the myth of battleship supremacy still reigned large at this time despite the increasingly obvious superiority of the aircraft carrier.
I'd better cut this long story short and say that Japan squandered its naval supremacy at the Battle of the Coral Sea (in which Australia took part) and the follow up Battle of Midway. After that Japan could no longer adequately defend itself in the Pacific which allowed the US to jump from place to place getting ever nearer to the Japanese mainland and launching massive air attacks which the Japanese had no answer to - and we all know the rest of the story?.....
