I can only give an unprofessional and brief summary, it just so happens I read about this while doing my own personal research on the Council of Yalta. Hopefully someone more knowledgeable on the matter can explain in more detail and correct any mistakes on my part.

Essentially, Poland had been invaded by both Germany and Russia, who divided up the country amongst themselves. The Polish government fled to Britain, and as you know, many joined the British army to fight against the Nazis and, one day, liberate their home. This includes Anders Army, which, from my understanding, was formed by Russia but fled South into Palestine and joined the British there, and like you said wound up in Italy.

However, during the council of Yalta, those governments in exile such as Poland and even France, were largely ignored by the "Big Three"- Churchill, Roosevelt, and Stalin. It was here that the allies agreed to let Stalin keep his holdings as per his prior agreement with Germany, so roughly half of Poland I believe.

The Soviets, as they pushed West during the War, had no intention of giving up anything they "liberated", in order to establish a Communist Eastern Europe.

Long story short, the Polish government in exile was pre-democracy, as well as most if its troops. They were considered "enemies of the state" by the new puppet government in Poland, with large scale imprisonment and executions of key leaders of the Polish resistance, including survivors of the Warshaw Uprising. They were slapped with charges of "crimes against the state" out of fear that they would cause problems with the new pro-soviet leadership. Widespread persecution of pro-democratic individuals and groups by the government became the norm.

This left the democratic Poles with nowhere to go, and as you said quiet a few committed suicide, and there was a general feeling of betrayal. The British government did however Offer blanket citizenship to some 200,000 Poles now displaced, a quick google reference brings up the Polish Resettlement Act of 1947.

This happened in other European countries as well. For those who are interested, the Council of Yalta also agreed that any Russian fighting for or living in Germany territory that was captured by the Allies would be handed over to the Soviets. This included families who had fled Russia during the revolution decades prior to escape communism.

Many Russians joined the German army to fight against Russia, with many captured Soviets simply switching sides, with hopes of overthrowing Stalin. A great many of then were placed on guard duty on the Western front, as German high command didnt trust them fully in the East. As a result, many were captured in the Normandy invasion, and were promptly turned over to the Soviets and either killed for treason or sent to labour camps.

All the Russian POWs, held in Britain and the US at the end of the war (many expecting they would be allowed to assimilate) were also handed over to a similar fate.

As a whole, the Council of Yalta was a distasteful affair. The Big Three had only their priorities in mind, and trusted Stalin far too much. This resulted in a lot of trouble and heartache for a great deal many people, from a great deal many places, including your Grandfather.

Hope this helps give a bit of an idea, maybe someone else can elaborate further.