Defence lawyers acting for Bradley Manning, the US soldier who fed a trove of state secrets to WikiLeaks, have called for several of the 22 counts against him to be dismissed, including the most serious charge that he "aided the enemy".

Manning's lead lawyer, the civilian attorney David Coombs, has filed four motions with the military court in Fort Meade, Maryland, asking the judge to drop several charges because of lack of evidence. In addition to aiding the enemy, the relevant counts include the allegation that Manning stole or purloined US property in the form of unauthorised intelligence drawn from Afghan and Iraq warlogs, Guantánamo detainee files and hundreds of thousands of US diplomatic cables from embassies around the world.

Coombs has also filed a motion to dismiss the allegation that Manning violated section 1030 of the federal Computer Fraud and Abuse Act by "knowingly exceeding authorised access" on a secret military network and transmitting documents to WikiLeaks, "with reason to believe that such information so obtained could be used to the injury of the United States, or to the advantage of any foreign nation".

Section 1030 carries a maximum sentence of 10 years, while aiding the enemy – a violation under military law – carries a possible life sentence with no chance of parole. Manning has already pleaded guilty to less serious offences carrying a maximum sentence of 20 years, although he has denied that he aided the enemy.

Coombs opened the defence stage of the trial of Manning by playing the court the video of a 2007 Apache helicopter attack on a group of civilians in Baghdad. The video is one of the most famed releases by WikiLeaks which posted it in April 2010 under the title Collateral Murder.

The defence had intended to screen 20 minutes of the footage focusing on the actual attack in which two Reuters journalists died. But the military prosecutors insisted that the entire 39 minutes of film was played to show the aftermath at the scene.

Coombs also entered into the court record an excerpt of The Good Soldiers by the Washington Post reporter David Finkel. The lawyer said that the excerpt was designed to show that the Apache video had not been "closely held" by the US government – in other words, it was not regarded as so secret as that it must never be made public.

In earlier hearings, Manning has revealed that he was partly motivated to leak the video to WikiLeaks after he noticed that Finkel's book contained a precise transcript of sections of the audio from the attack helicopter. He said that he realised that the journalist must already have been provided with the raw footage.

Manning's defence comes after five weeks of intermittent government evidence in a trial that will go down in US history as the highest-level prosecution of a leaker of state documents in at least a generation. Coombs is expected to call about 40 witnesses, potentially taking the trial into August.

Among the first 10 defence witnesses, whose names have been made public, are the former chief prosecutor at the Guantanamo detention camp, Colonel Morris Davis. One of the 22 counts against the soldier relate to his leaking of more than 700 files of Guantanamo detainees.

Professor Yochai Benkler, a Harvard law professor who heads the Berkman Center for Internet and Society, will also be called later in the week. It is expected that his testimony will focus on the role of Wikileaks as a modern digital organisation, the defence hope being to undermine the prosecution claim that Manning intentionally and "wantonly" sought to assist enemy groups, notably al-Qaida, by leaking the material.

The defence's room for manoeuvre in questioning witness is very tightly curtailed. The judge, Colonel Denise Lind, who is presiding over the case in the absence of the jury, has ruled in earlier sessions that the defence cannot raise Manning's motives for leaking or attempt to show how harmless the leaks were to US interests until the sentencing phase of the trial.

The first defence witness called on Monday was chief warrant officer Joshua Ehresman, who worked alongside Manning in the intelligence unit of Forward Operating Base Hammer outside Baghdad between November 2009 and May 2010. Ehresman described the culture of the unit as one in which intelligence analysts such as Manning regularly downloaded classified information onto CDs, as well playing music and movies on their secret government computers. There were no rules on what an analyst could or could not burn onto a CD or download from the secret Siprnet database to which they had access.

Ehresman described Manning as the best intelligence analyst of the group. "He was the best; that's why he was our go-to guy for that stuff," he told the court.