💡 Detailed Explanation
"～あってこそ" (atte koso) is a grammatical form used to emphasize that something can only be achieved or established *because* of something else. In this sentence, it means "It was precisely because of his many years of effort that he finally achieved his dream." "～あっての" (atte no) modifies a noun, meaning "A that can only exist because of B" (e.g., "家族あっての私" - I exist because of my family). "～にあって" (ni atte) means "in a situation/position of ~" or "under ~ circumstances." "～とあって" (to atte) indicates a reason or cause, meaning "because there are circumstances of ~."