I assume you talk about the invasion of Poland. The relations between Poland and Germany were very complex. Poland as a nation was "created" after World War 1 as second Polish republic and included many areas which previously were part of Germany/Prussia. The city Danzig became a free city and parts of Eastern Prussia remained in German hands ( as exclave). This created obviously potential for friction between the two countries and the right wing in Germany was using the Polish issue from the very beginning as talking point. Many Germans were still living in Polish territory which made it even easier for the right wing. This situation allowed for "clever" propaganda and politics. Disputes between the two countries existed long before the NAZIs came to power tho.

Those tension increased with Hitlers take over, his "living space in the east" view point was already laid out in Mein Kampf. While Poland was never specifically mentioned as target it could understandably feel threatened by the new German government. This culminated in a partial mobilization of the Polish army.

Over the next years Hitler tried to appease the Poles which was quite successful and resulted in closer ties between the countries.

Germany toughens their stance:

While Germany geared up for war it started to increase the pressure on Poland and demanded more concessions. The major focus was the free city Danzig and a direct connection to her Königsberg exclave. The offered concessions were minor. Also, Germany tried to isolate Poland from its Allies. Poland was now in a tough spot, the relationship with France was already deteriorated but bending to Germanys will would relegate the country to satellite status.

After that, Germany "occupied" Czechoslovakia which made the Polish situation even worse. Poland itself was operating aggressively against parts of Czechoslovakia during the 20s and was now completely isolated between Germany and the USSR with both these countries it was unable to solidify treaties.

From there on it went south and the UK, which now felt Germany was a growing threat, declared it will support Poland in the case of war. The talks between Germany and Poland more or less stopped.

In the spring of 1939 Hitler killed the non aggression treaty and already in April attack plans on Poland were drawn up.

to your question

What spin did the Nazis put on it

Comparable to recent events in eastern Europe the German propaganda started to report from Polish acts of aggression against Germans living in Poland. The propaganda started immediately with the breaking down of the talks. Like with every major propaganda campaign major target was creating a "Feindbild" the illusion of an enemy an existential threat. This culminated in the Gleiwitz incident. A false flag operation of the SS. During this operation and several others, German personnel simulated Polish attacks on the German border. Those false flag operation all were used to create a casus belli. Immediately after those operations, Hilter ordered his armies to invade Poland.

He addressed the German people and Reichstag and explained how this was a mere defensive move against an aggressive Poland.

September 1st ( "Declaration of war" )

Polen hat heute Nacht zum ersten Mal auf unserem eigenen Territorium auch mit bereits regulären Soldaten geschossen. Seit 5:45 Uhr wird jetzt zurückgeschossen! Und von jetzt ab wird Bombe mit Bombe vergolten! Wer mit Gift kämpft, wird mit Giftgas bekämpft. Wer selbst sich von den Regeln einer humanen Kriegsführung entfernt, kann von uns nichts anderes erwarten, als dass wir den gleichen Schritt tun. Ich werde diesen Kampf, ganz gleich, gegen wen, so lange führen, bis die Sicherheit des Reiches und bis seine Rechte gewährleistet sind.

Translation:

Last night for the first time Polish regular soldiers invaded our territory and attacked from our soil. We have now been returning fire since 5:45 a.m.! )Henceforth, bomb will be met with bomb. He who fights with poison shall be fought with poison gas. He who distances himself from the rules for a humane conduct of warfare can only expect us to take like steps. I will lead this battle, whoever the adversary may be, until the security of the Reich and its rights have been assured.

The bold part reads strongly as "now we retaliate" implying this is an act of defense.

The foundation for the propaganda was laid long before 1939 and was supported by the fact the Germany lost territory to Poland due to the Versailles treaty.

Some more examples out of Hitlers speech give you a good understanding of how Poland was included in the general Nazi agenda of nullifying the Versailles treaty which was a propaganda cornerstone, maybe the biggest.

Danzig was torn from us! Poland annexed the Corridor along with other German territories in the East! The German minorities living there are being persecuted in the vilest manner imaginable. Already in the years 1919 and 1920 over one million ethnic Germans were forced off their lands.

You see the similarities to other conflicts in history? Wrongfully taken from the Germans...

As always, I also sought here to bring about a change by peaceful means, by offering proposals to remedy this situation which meanwhile had become unbearable... You know that all these proposals were rejected.

Germany was left with no choice, it was trying to act peacefully. He means the negotiations over Danzig and the corridor both were significant demands.

In Germany during this time, the media was controlled by the NSDAP and all major news outlets brought the same coordinated message. Hitlers speech exemplifies what the normal German at this time would have read or heard about the Polish issue for the last months.

Like I said before special attention was given to Germans living in Polish territory:

The minorities living within the Reich are not subject to persecution. There is not one Frenchman who can stand up to claim that the 50,000 to 100,000 French living in the Saar territory are being oppressed, tortured, or robbed of their rights. Not one can claim as much! I have been silently standing by and observing the situation for four months now. Yet I have issued warnings repeatedly. And I have intensified these warnings of late.

To summarize the position which was presented to the German people: Germany tried its best to negotiate with Poland which was unreasonable and replied with economic attacks on Danzig and refuted all peaceful attempts for solutions. The Polish then started to escalate the situation by attacking the German border and threatening the free city of Danzig while also wrongfully prosecuting Germans living in Polish territory. This situation was created by the Versailles treaty in the first place and had to be corrected to assure the safety of the German people.