Question: What prompted France and the UK to declare war on Germany in 1939
About the article "Invasion of Poland", section "Abstract".

Candidate passages:
 in World War II.
Poland partly mobilized its troops on August 24, 1939, and fully mobilized on August 30, 1939, following the increased confrontations with Germany since March 1939.
On September 1, 1939 Germany invaded Poland, which prompted both France and Britain to declare war on Germany.
However, they were slow to mobilize, and by the time Poland had been overrun by the Axis powers, only minor operations had been carried out by the French at the Saar River.
Canada actually carried out a partial mobilization on 25 August 1939. in anticipation of the growing diplomatic crisis.
On September 1, 1939, the Canadian Active Service Force (a corps-sized force of two divisions) was mobilized even though war was not declared by Canada until September 10, 1939.
Only one division went overseas in December 1939, and the government hoped to follow a "limited liability" war policy.
When France was invaded in May 1940, the Canadian government realized that would not be possible and mobilized three additional divisions, beginning their overseas employment in August 1940 with the dispatch of the 2nd Canadian Division (some units of which were deployed to Iceland and Newfoundland for garrison duty before moving to the UK).
Canada also enacted the National Resources Mobilization Act in 1940, which among other things compelled men to serve in the military, though conscripts mobilized under the NRMA did not serve overseas until 1944.
Conscripts did, however, serve in the Aleutian Islands in 1943 though the anticipated Japanese defense never materialized due to the evacuation of the enemy garrison before the landings.
Service in the Aleutians was not considered "overseas" as technically the islands were part of North America. 
World War II.
Nazi Germany.
After Nazi Germany invaded Poland on September 1, 1939, the United Kingdom and France declared war on September 3.
To assert Canada's independence from the UK, as already established by the Statute of Westminster 1931, Canada's political leaders decided to unnecessarily seek the approval of the federal parliament to declare war.
Parliament was not scheduled to return until October 2, but returned to session early on September 7 to consider the declaration of war.
The Senate approved a declaration of war on September 8 and the House of Commons approved it on September 9.
The following day, Prime Minister Mackenzie King and the Cabinet drafted an Order in Council to that effect.
Canadian diplomats brought the document to King George VI, at the Royal Lodge, Windsor Great Park, for his signature, whereupon Canada had officially declared war on Nazi Germany.
In his capacity as the government's official recorder for the war effort, Leonard Brockington noted: "King George VI of England did not ask us to declare war for him—we asked King George VI of Canada to declare war for us."
 27 February - United Kingdom and France recognize Franco's government in Spain.
17 June - Last "public" guillotining in France - murderer Eugen Weidmann.
3 September - United Kingdom, France, New Zealand and Australia declare war on Germany.  
The “cash and carry” legislation passed in 1939 was not a failure, it simply was not going to be an effective measure after Germany began invading its neighbors.
After the fall of the Low Countries (Belgium and the Netherlands) and the invasion and capitulation of France, Roosevelt lobbied for the introduction of Lend-Lease.
Further, the concept of “cash and carry” had enabled Japan to continue lots of trade with the US while China’s trade had dropped to practically nothing, given their slim economic resources.
Helping an enemy at the expense of an ally was not the effect Roosevelt had been hoping for.
It also affected Italy.
Lend Lease was a plan in which the European allies didn't have to pay cash or arrange transportation any longer.
Instead, the U.S. would demand payment at a later time.
In keeping with the Monroe Doctrine, the U.S. did not actively participate in the war until after the attack on Pearl Harbor and the Japanese declaration of war prompted Congress to reciprocate and declare war on Japan, after which Italy and Germany declared war on the U.S.
The U.S. then switched from allied assistance to active engagement. 
France was one of the original guarantors of Polish security and as such was one of the first countries to declare war on Germany.
In 1940, following the Battle of France, the French government signed an armistice with Germany, leading to the foundation of Vichy France and Free French Forces in exile.
The leader of the Free French, Charles de Gaulle, took control of France in 1944 and the country ended the war as an ally.