Why did Solomon (peace be upon him) address the Queen of Sheba and her people with a threat, saying: "Do not exalt yourselves above me, but come to me in submission" (Quran 27:31)? And why did she believe after witnessing his power? And is what he did considered spreading monotheism by the sword?

The message of Prophet Sulaiman (peace be upon him) to the Queen of Sheba was not a threat; rather, he began it with "In the name of Allah, the Most Gracious, the Most Merciful," and invited them to monotheism and Islam, forbidding them from arrogance. The Queen knew that when kings enter a village, they ruin it, and she wanted to ascertain Sulaiman's true nature before submitting to him. Sulaiman's goal was to invite the Queen and her people to monotheism and to abandon polytheism. Matters are judged by their outcomes, and the Queen submitted to Allah, the Lord of the Worlds. The sword did not spread Islam by compulsion, but it removed obstacles in the path of the true religion.