What is an invasion?

Texas is trying to invoke the Guarantee Clause which requires the federal government to provide protection against foreign invasion. They claim that the federal government is not doing anything to protect them against an invasion of illegal aliens and drug smugglers.
The democrats are claiming there is no invasion because the illegal aliens are not armed and are not a part of an invading army.
There have been over 100,000 deaths from fentanyl overdoses during 2021, the leading cause of death for Americans between the ages of 18 and 45. It this statistic alone enough to call this situation an invasion?
Is this an invasion?
Is this an invasion?
What is an invasion?
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