Theresa May has used her Christmas message to praise the work of the Armed Forces, particularly in Syria and after the nerve agent attack in Salisbury.
The PM said UK forces had "continued to demonstrate why you are the finest in the world".
Meanwhile, Labour leader Jeremy Corbyn paid tribute to those who emulate the Good Samaritan in his Christmas message.
He drew on the Christian parable to applaud those who help others.
In her message, Mrs May said the UK's Armed Forces had played a "vital role" in cleaning up after the Novichok attack, protecting the country's waters and skies from Russian intrusion, and also elsewhere in the fight against the Islamic State group and terrorism.
They had also helped, alongside US and French allies, to send a message to the Assad regime in Syria "that we will not stand by while chemical weapons are used, as they were in April on families, including young children".
"Time and again, you have stood up to aggression and those who flout the rules-based international order," she said.
"You should be incredibly proud of all that you do, just as the whole country is proud of you."
The military was called in to Salisbury after the nerve agent attack in March, to help remove vehicles and objects that might have been contaminated.
Mrs May added: "Now as we approach the new year, there will be new challenges ahead.
"But I know you will continue to meet them in the same way that you have always done. With courage, determination, resilience, ingenuity.
"Qualities that are as vital now as they have ever been."
In a video to run on social media, Mr Corbyn said that he saw "the compassion of the Good Samaritan in people across our country every day but especially at Christmas".
"Whether it's people helping in homeless shelters, offering comfort and meals for those looking to escape the freezing temperatures and dangers of sleeping rough.
"Those ensuring food banks are fully stocked for people who can't afford to eat properly this Christmas.
"Or volunteers raising money for refugees who've been forced to flee war, oppression and devastation.
"These are people who will not 'walk by on the other side'.
"They do what's become so necessary in a system that is failing to provide for people's basic needs."
He said these people "embody what's best and most compassionate in all of us".
"They make me certain that we can build a fairer society which works for everyone."
Mr Corbyn's message comes after charity Crisis said homelessness in the UK was at a record high - with 170,000 families and individuals experiencing destitution.