Final answer:
Islam's expansion into Europe was viewed as threatening by European rulers, leading to conflicts such as the European crusades.
Explanation:
Threatening accurately describes how rulers in Europe viewed the expansion of Islam.
Islamic expansion into Europe was often met with opposition, as seen through events such as the raid on Rome by Aghlabid emirs in 846.
The European crusades were a response to Islamic rule, showcasing the tensions between Islamic empires and various Christian kingdoms.
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