They had a much larger force planned, but they attacked the USMC at the Tenaru (USMC RGT) because they believed they could overrun what they thought was a screen/patrol not a dug in prepared defense in RGT strength. They were cut down like they ran into a chainsaw. Later Japanese Forces just met more dug in and better prepared US forces. John Toland's book goes into great detail about how the Japanese tried to flank the US beachhead by carrying even disassembled artillery pieces single-file thru the mountains - most of which were discarded on the route and are still there to this day. It took much, much longer than planned, and still didn't crack the US defense. They continued to pour more and more troops in, but NEVER had even parity with the US.