The phrase "buckled under" refers to yielding or succumbing to pressure. The opposite of this phrase is to stand firm or be resolute in the face of such pressure. Words that can be used as antonyms for "buckled under" includes phrases like "persevere," "endure," "resist," "persist," "persist," "prevail," and "overcome." These words denote the ability to withstand or rise above challenging situations without giving in or giving up. While "buckled under" may signify weakness, the antonyms denoting strength and determination can help us stay motivated and focused in the face of adversity.