The phrase "51st state" can be used in a positive sense, meaning that a region or territory is so aligned, supportive, and conducive with the United States, that it is like a U.S. state. It can also be used in a pejorative sense, meaning an area or region is perceived to be under excessive American cultural or military influence or control. In various countries around the world, people who believe their local or national culture has become too Americanized sometimes use the term "51st state" in reference to their own countries. 

Under Article IV, Section Three of the United States Constitution, which outlines the relationship among the states, Congress has the power to admit new states to the union. The states are required to give "full faith and credit" to the acts of each other's legislatures and courts, which is generally held to include the recognition of legal contracts, marriages, and criminal judgments. The states are guaranteed military and civil defense by the federal government, which is also obliged by Article IV, Section Four, to "guarantee to every state in this union a republican form of government".
Is 51st state positive or negative?
What's an example of a positive use?
How about a negative use?
Who has the ability to actually make one?
What gives them that ability?
Would the new state be given assured protection?
What else?
What would they be required to give in return?
What will the 51st state be?
Would a marriage in Texas need to be honored in the 51st state?
Both
The phrase "51st state" can be used in a positive sense, meaning that a region or territory is so aligned, supportive, and conducive with the United States, that it is like a U.S. state. It can also be used in a pejorative sense
a region or territory is so aligned with the United States, that it is like a U.S. state
a region or territory is so aligned, supportive, and conducive with the United States, that it is like a U.S. state
an area or region is perceived to be under excessive American cultural or military influence
pejorative sense, meaning an area or region is perceived to be under excessive American cultural or military influence
Congress
Congress has the power to admit new states to the union
the United States Constitution
Under Article IV, Section Three of the United States Constitution, which outlines the relationship among the states, Congress has the power to admit new states to the union
Yes
The states are guaranteed military and civil defense by the federal government, 
a republican form of government
also obliged by Article IV, Section Four, to "guarantee to every state in this union a republican form of government".
"full faith and credit" to the acts of each other's legislatures
The states are required to give "full faith and credit" to the acts of each other's legislatures
unknown
unknown
Yes
include the recognition of legal contracts, marriages, and criminal judgments.