"Why such daily cast of brazzon cannon and foreign mart for implements of war?"
When the "valiant Hamlet (Sr.) did slay this Fortinbras," a conflict began to arise between Denmark and Norway. Denmark gained new lands, but will this action cause trouble for them in the future?
Meanwhile...
Join my army, you criminals
"Now, sir, young Fortinbras, of unimproved mettle hot and full" wants to get revenge on Denmark and gain his lands back. He is very impulsive and angry, and getting land back and revenge is top priority in his mind.
Peace Keeping
"He hath not failed to pester us with message...To business with the King more than the scope of these dilated articles allow"
Marcellus is wondering why the castle seems like they are preparing for war. None of the citizens of Denmark have been told what is going on, but they have been on constant guard shifts, have been importing weapons, and have been making many weapons.
The Deal
"Fortinbras...Makes vow before his uncle never more to give th' assay of arms against your Majesty"
Young Fortinbras has "sharked up a list of lawless resolutes" to fight a war against Denmark. He is making an army full of criminals to regain lands back. He is gearing up to invade!
King Claudius is explaining in a speech to the people of Denmark that Norway had been waging war against them but has decided to try to keep peace instead. He is sending a messenger to Norway to persuade them not to go to war.
After the messenger comes back from talking to Young Fortinbras's uncle, he relays the message that Norway has agreed not to invade Denmark. They make an agreement instead and the war is off. Hooray!