There was no declaration of war to invade Grenada.
there was no declaration of war in Afghanistan.
There was no declaration of war in Vietnam.
There was no declaration of war in Korea.
President Obama did not seek approval of the Congress in the attacks on Libya.
Korea and Vietnam were both before the War Powers Act.
President Bush 1 had Congressional Approval before Desert Storm. (Jan 12, 1991)
President Bush 2 did seek approval of both houses of Congress on both Iraq and Afghanistan conflicts.
On October 10, 2002, the House of Representatives voted 296-133 to authorize President Bush
to use force against Iraq. Later that evening, the Senate joined the House in supporting the measure
by a vote of 77-23.
In Grenada President Reagan's actions were "legal," since section 4(a)(1) of the War Powers Act
gives the executive a 60-90 day time limit on troop action (without further congressional approval.
All US forces were out of Grenada within that time frame.